Playback feture not availible

hi.i have a emerson ewd2003 dvd player and some disk i have ran intohave loaded and said error playeback feature may not be availible on this disk.i put it in another player in my sons room and they play fine.this started about 6 months ago.i have about eight dvds that do this.not all new disk,i just run into one every now and then.i was thinking maybe a firmware issue or something,maybe some new anticopying measure that is messing with it?these are dvds i bought at walmart.any ideas? thanks