necvox 6 cd/dvd changer?

I have a NECVOX 6 CD/DVD Changer installed in my car with headrest mounted Alpine TV's. The problem is that the changer will play CD's with no problem but when I try to play a DVD I get the message on the screen "Wrong Disk". I have tried all types of DVD's with different regions and even region free to no avail. Any help would be greatly appreciated. The fact that it plays CD's indicates to me that the problem can't be too severe. I'm thinking that it may be a setting, although I have checked all of the on-screen settings.