DirectX - Direct Show programming?

Hi,
I'm Developing a live video streaming application in vc++ using DirectX VideoMixingRenderer filter. I can able to draw rubber band rectangle on live video using DirectX SetAlphaBitmap() through my mouse move. if my video frame size(4CIF) is greater than my picture box control size, it's working fine. Suppose, my video frame size(CIF) is less than my picture box control size,
1) only 3 sides of the rectangle can be visible.
2) sometimes 2 sides only can be visible.
3) Above two context has been happen randomly.
Please, somebody help me out from this issue.

Thanx in advance,
Regards
Surez