That's very, very hard to do without knowing exactly what your workflow is. Not to mention that if you were working in Avid, I could easily help you, but if you were using Vegas I wouldn't even know where to begin. It depends on a lot of different variables.

The more we know about what codecs you used in the editing process, etc., the better we can help.

Oh, and something I just thought of - one thing that almost always happens to me in Avid if I'm using multiple video tracks, is that I forget to have them all active. In Avid, if you have, say, V1 (video track 1) selected, but V4 isn't, then it will only render out V1, ignoring V2, V3, and V4. I have to have the V4 monitor on for all of them to render.

Make sure everything is selected before you render, just in case.