Masking Error. Like Wtf

[MycathatesyouAMV]

Ok as for all of u vegas users u all know that there is a small screen that previews your video. In masking which I have been doing frame by frame, the masking screen is like always set off 1 frame from the preview screen, which is the actually frame. Dont know why its like this. And sometimes Like If I mask out to many straight frames, the next one wont work, this is always sometimes.