Some dvds dont work on my ps3?

hey i was just wondering if anyone could tell me why an if its normal that i cant watch some dvds on my ps3.

its on an hd screen and blurays always work but some dvds, and they arent dodgy copies or anything, do not.

they come up in video menu saying DVD and when i try to watch it the screen goes black there is no sound and i have to turn the ps3 off at the back switch

thanks for any advice

Maybe it's the region code that don't match. The PS3 is Bluray movies, PS2 games and DVD movies Region locked (PS3 games are Region free), you can only play the disks that match with the region on your console. Check in the cover of the DVD that's not working which region is it, if your PS3 is from US it is a Region 1 code. If the problem is not region, i don't know what it could be.