The disc changer maybe out of alignment. Ask someone who knows what they are doing to take off the upper casing and have a look inside.
I myself disassembled the top part. Plugged it back in then tried all the discs, changing them around until I identified the problem. I also used the vacuum cleaner to remove the dust, and a cotton bud
(q-tip) to clean the laser eye. Once everything was working I screwed it back together while it was still playing.
Slightly dangerous I know, but I wore rubber gloves the whole time.
Hope this helps. I was searching for ages for a fix to this, so I am posting this to help anyone else who may be looking.