Sometimes even region free discs have trouble playing if they are encoded at 50hz, or have PAL SD extras. Usually if the extras are PAL the player will just skip them or not play them, if they can't convert them to 60hz.
Sony players, while great at playing Blu-rays, are some of the worst with import discs.
The US PS3 is locked at Region A for Blu-ray, and Region 1 for DVD. It want convert or play 50hz content, or PAL SD extras.