I was wondering if anybody had any ideas as to why my cd changer will sometimes play mp3's because this is my problem

I burned a cd with mp3's on it and put into slot 2 (slot 1 had a regular cd in it) and it worked so I made 3 more mp3 cd's into slot 4,5,6 and I selected slot 6 and it played fine then I tried to play slot 5 and it locked up and I opened the changer face and it would not go back down, eject cd or anything. I could only listen to radio until I shut the car down and then it would close and start all over again, I have been able to play slot 2 once and slot 6 once nothing after that ??? Any ideas or help would be greatly appreciated. Thanks Sean