some DVD disc may not play in my DVD player why?

Are these DVDs maybe from different countries than your own? If they are, they may be coded for a different region. If that is the case, you can probably reprogramme your DVD player to play disks from any region. Just go onto a search engine and type the make of your DVD player plus "multi region" and there's a pretty good chance you'll find that someone has posted instructions on how to reprogramme your DVD player. Obviously this doesn't help if you're inadvertently trying to use Blue Ray disks.

you might have accidently gotten blue ray disc. I went to the library and got some dvds a few weeks ago and the one I really wanted to watch would not work and I found out that it was blue ray!!