This has never happened tp me before! Main video on one track. A 2nd (higher) track has some video overlay files. A third track has titles. On rendering (both MPEG-2 (for DVD) and AVC (BR) completely (and randomly!!!) passes by some of the overlay videos! Subsequent renderings have different ones missing! Titles work fine. No fancy edits in overlay videos; just some color correction and transitions (barn doors).