I had the same problem. When I bought a Galaxy, the changer had two CD's stuck in it and an error was displayed. I took it apart and got it working but the same thing happened.
I found out after that this is a common problem when the battery goes flat with discs in the changer. As far as I can tell, the changing mechanism can't find a home position and becomes misaligned.
You can try taking it aprt and having a look or doing what I did. Ford wanted about 250 quid IIRC. A smaller local firm did an exchange unit for 120. I replaced it about 3 years ago and it's been fine.

HTH

Shanny