A few wild guesses...

Clamping magnet is so strong that the load/eject drive can't overcome
it's grip. This was a problem in some Onkyos, the remedy was an
adhesive spacer sheet to separate the chucking pieces. With a disc
present, the magnet and its companion are separated by the CD itself.

I've seen some units focus only when sideways or upside down. Trouble
was in pickup which had "sagging optics" requiring more voltage to lift
into focus. Maybe you could fudge it a little by lowering the disc
table height.

Good Luck,

Doug