Jim Simon wrote:Your Track Destination wasn't set for video, just audio. That could be relevant.
BINGO!!
I had already exported the new timeline, but I'm sure someone will google this in 3 years and it will save them lots of trouble.
This was 100% the problem.
For anyone in the future, click the greyed out "V1" in the video track - or "A1" if you're issue is reversed - that should do the trick.