This is lifted from the Adobe forums.
“I’m writing back just to let people know that Neil Wilkes was exactly right, at least for the problem I was encountering: the issue was that I needed to re-size each video/audio pair to make sure they were exactly the same duration. It did come down to a 1 FRAME difference in one of the sequences, so it is important to ZOOM ALL THE WAY IN before you start determining that if the lengths match up or not.
I don’t know why the clips would not import at exactly the same duration as when I used Premiere to create them…very odd…nonetheless, thanks Neil for the useful tip, and thanks John for pointing me in its direction.”