On most slot loading CD players there are usually a pair of rollers and as the disc lifts off the spindle, the bottom roller lifts and grips it against the upper roller and then it's powered out of the slot.
Anything contaminating the rollers making them slippery can effect this, another possibility is the springs that exert the pressure on the disc rollers becoming weak.
All very uncommon problems.
The only other possibility I can think is that something inside the mechanism is obstruction the disc coming out, I seem to think fiat have a couple of pins that come down and stop any other discs being put it, maybe these aren't lifting properly