问题的出现原因：

可能在处理多点触控/绘图时遇到了问题。

解决方法：

参考Android开发者网站和Android博客上关于处理多点触控/绘图的教程。

根据这些教程，我创建了一个示例，我认为这个示例与您尝试实现的内容相似（不完整的圆形绘制 - 圆形由单个触摸生成）。

代码如下：

public class CirclesDrawingView extends View {
    private static final String TAG = "CirclesDrawingView";
    /** Main bitmap */
    private Bitmap mBitmap = null;
    private Rect mMeasuredRect;
    
    /** Stores data about single circle */
    private static class CircleArea {
        int radius;
        int centerX;
        int centerY;
        
        CircleArea(int centerX, int centerY, int radius) {
            this.radius = radius;
            this.centerX = centerX;
            this.centerY = centerY;
        }
        
        public String toString() {
            return "Circle[" + centerX + ", " + centerY + ", " + radius + "]";
        }
    }
    
    /** Paint to draw circles */
    private Paint mCirclePaint;
    private final Random mRadiusGenerator = new Random();
    // Radius limit in pixels
    private final static int RADIUS_LIMIT = 100;
    private static final int CIRCLES_LIMIT = 3;
    /** All available circles */
    private HashSet mCircles = new HashSet(CIRCLES_LIMIT);
    private SparseArray mCirclePointer = new SparseArray(CIRCLES_LIMIT);
    
    /**
     * Default constructor
     *
     * @param ct {android.content.Context}
     */
    public CirclesDrawingView(final Context ct) {
        super(ct);
        init(ct);
    }
    
    public CirclesDrawingView(final Context ct, final AttributeSet attrs) {
        super(ct, attrs);
        init(ct);
    }
    
    public CirclesDrawingView(final Context ct, final AttributeSet attrs, final int defStyle) {
        super(ct, attrs, defStyle);
        init(ct);
    }
    
    private void init(final Context ct) {
        // Generate bitmap used for background
        mBitmap = BitmapFactory.decodeResource(ct.getResources(), R.drawable.up_image);
        mCirclePaint = new Paint();
        mCirclePaint.setColor(Color.BLUE);
        mCirclePaint.setStrokeWidth(40);
        mCirclePaint.setStyle(Paint.Style.FILL);
    }
    
    public void onDraw(final Canvas canv) {
        // background bitmap to cover all area
        canv.drawBitmap(mBitmap, null, mMeasuredRect, null);
        for (CircleArea circle : mCircles) {
            canv.drawCircle(circle.centerX, circle.centerY, circle.radius, mCirclePaint);
        }
    }
    
    public boolean onTouchEvent(final MotionEvent event) {
        boolean handled = false;
        CircleArea touchedCircle;
        int xTouch;
        int yTouch;
        int pointerId;
        int actionIndex = event.getActionIndex();
        
        // get touch event coordinates and make transparent circle from it
        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_DOWN:
                // it's the first pointer, so clear all existing pointers data
                clearCirclePointer();
                xTouch = (int) event.getX(0);
                yTouch = (int) event.getY(0);
                // check if we've touched inside some circle
                touchedCircle = obtainTouchedCircle(xTouch, yTouch);
                touchedCircle.centerX = xTouch;
                touchedCircle.centerY = yTouch;
                mCirclePointer.put(event.getPointerId(0), touchedCircle);
                invalidate();
                handled = true;
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                Log.w(TAG, "Pointer down");
                // It secondary pointers, so obtain their ids and check circles
                pointerId = event.getPointerId(actionIndex);
                xTouch = (int) event.getX(actionIndex);
                yTouch = (int) event.getY(actionIndex);
                // check if we've touched inside some circle
                touchedCircle = obtainTouchedCircle(xTouch, yTouch);
                mCirclePointer.put(pointerId, touchedCircle);
                touchedCircle.centerX = xTouch;
                touchedCircle.centerY = yTouch;
                invalidate();
                handled = true;
                break;
            case MotionEvent.ACTION_MOVE:
                final int pointerCount = event.getPointerCount();
                Log.w(TAG, "Move");
                for (actionIndex = 0; actionIndex < pointerCount; actionIndex++) {
                    // Some pointer has moved, search it by pointer id
                    pointerId = event.getPointerId(actionIndex);
                    xTouch = (int) event.getX(actionIndex);
                    yTouch = (int) event.getY(actionIndex);
                    touchedCircle = mCirclePointer.get(pointerId);
                    if (null != touchedCircle) {
                        touchedCircle.centerX = xTouch;
                        touchedCircle.centerY = yTouch;
                    }
                }
                invalidate();
                handled = true;
                break;
            case MotionEvent.ACTION_UP:
                clearCirclePointer();
                invalidate();
                handled = true;
                break;
            case MotionEvent.ACTION_POINTER_UP:
                // not general pointer was up
                pointerId = event.getPointerId(actionIndex);
                mCirclePointer.remove(pointerId);
                invalidate();
                handled = true;
                break;
            case MotionEvent.ACTION_CANCEL:
                handled = true;
                break;
            default:
                // do nothing
                break;
        }
        
        return super.onTouchEvent(event) || handled;
    }
    
    /**
     * Clears all CircleArea - pointer id relations
     */
    private void clearCirclePointer() {
        Log.w(TAG, "clearCirclePointer");
        mCirclePointer.clear();
    }
    
    /**
     * Search and creates new (if needed) circle based on touch area
     *
     * @param xTouch int x of touch
     * @param yTouch int y of touch
     *
     * @return {CircleArea} obtained circle
     */
    private CircleArea obtainTouchedCircle(final int xTouch, final int yTouch) {
        CircleArea touchedCircle = getTouchedCircle(xTouch, yTouch);
        if (null == touchedCircle) {
            touchedCircle = new CircleArea(xTouch, yTouch, mRadiusGenerator.nextInt(RADIUS_LIMIT) + RADIUS_LIMIT);
            if (mCircles.size() == CIRCLES_LIMIT) {
                Log.w(TAG, "Clear all circles, size is " + mCircles.size());
                // remove first circle
                mCircles.clear();
            }
            Log.w(TAG, "Added circle " + touchedCircle);
            mCircles.add(touchedCircle);
        }
        return touchedCircle;
    }
    
    /**
     * Determines touched circle
     *
     * @param xTouch int x touch coordinate
     * @param yTouch int y touch coordinate
     *
     * @return {CircleArea} touched circle or null if no circle has been touched
     */
    private CircleArea getTouchedCircle(final int xTouch, final int yTouch) {
        CircleArea touched = null;
        for (CircleArea circle : mCircles) {
            if ((circle.centerX - xTouch) * (circle.centerX - xTouch) + (circle.centerY - yTouch) * (circle.centerY - yTouch) <= circle.radius * circle.radius) {
                touched = circle;
                break;
            }
        }
        return touched;
    }
    
    protected void onMeasure(final int widthMeasureSpec, final int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        mMeasuredRect = new Rect(0, 0, getMeasuredWidth(), getMeasuredHeight());
    }
}

Activity中只包含`setContentView(R.layout.main)`，其中main.xml的内容如下：

这样就可以在Android中实现可拖动的圆形画布。