DVD player on a 2011 21 inch iMac will not play certain new DVD's.

I have run into a problem recently. Certain new dvd's will not play in my iMac dvd player. I have a 2011 21 inch iMac running Sierra. It will not recognize or load some new dvd's or start the DVD player app and then just eject the dvd. Is this because of the type of drive my computer has? Copy Protection on the dvd? Is there a way to override this? I've found the dvd's will play on a friend's pc and regular dvd player. But not on my mac. Can I get an external drive that will read it? I hate that a dvd set I just bought will not play on my computer as it's the only way I have to view them and rarely have issue with any other dvd's. But every once in a while one will not play at all. It's as if the drive won't recognize it or something is blocking it from loading. Has anyone else had this problem? Is it just my drive? Is it the software? OS Sierra? Thanks, Derek