white frames on H.264 clips
 

I have to often make H.264 clips for my composer who works on a Mac. Every time he gets a clip from me he has to chop off 2 or 3 white frames at the head of the clip, otherwise he sees no picture. He says that I am the only one who gives him these weird H.264's.

I am also the only one of his clients who edits with Vegas. Has anyone come across this problem and does anyone have a suggestion what I could do to stop this from happening, such as change a setting somewhere?

Thanks in advance,