Strange. Could you post or send me:

1. your input file
2. a sample log data file

I'll take a look and see what I can come up with. My initial guess is that DirectRT is not getting enough time to preload the clips. Sounds like you are using a 500ms inter-trial interval, yes? If this is not enough time to load the videos, then they will not be able to play at trial onset. Just out of curiosity--try a trial interval of 1000-2000ms and see if that affects the likelihood of proper display.