Timing of slides - not running for programmed time.

I posted/uploaded about ten items to play in rotation. Set each to run for 8 seconds. Never had a problem before, but noticed a few of the latest uploads were not running for the 8 seconds as programmed. One was barely up for 2 seconds. Reloaded the "problem" slides and had the same problem. Does anyone know why certain slides uploaded and programmed for 8 seconds weren't playing nice with the programmed time?