6 disc changer won't play burnt cd's

I have a 2002 zts with the 6 disc changer. It plays only a few of my burnt cd's. the rest skip or sound fuzzy and crackle. I know sometimes this happens with cd players to keep people from playing their illegal copies. is that the problem here? it plays all the discs that i purchase at stores just fine. if you go online and buy music that you pay for should those always work? or is it an issue of the focus liking cd-r as opposed to cd+r or vice versa. thanks for any help!!!

never heared of a cd+r

just burn the Cd at like 2x or 4x on high quality media