Could be any number of things. Most likely the combined bitrate is too high. If it's not that, it could be that the source had two different framerates. Or aspect ratios. Or the settings of the export could have any number of flags. I recommend checking those places first, and see if it resolves. I've often had to mux a film over a dozen times before I'm happy with it. I'll get all the way through previewing it and see a bad frame here or something off there and have to go back to the project file, make the trim or settings change, duplicate it in the audio, re-export the streams and re-mux the disc.

Blade Runner 2008 darn near killed me ... haven't done an edit in six months as a result, because I really wanted that one to be right and at the same time make fans as happy as possible with all the audio options.

In my early days of editing my problems were technical ones, not unlike what you're talking about. Hopefully the points in paragraph 1 will help you out.

--ADM