I have the same problem on my mac mini. Looking at it open, whilst running, I could see that the spindle is not releasing the disc. It's either sticking on to it more than it used to, OR (as I suspect) the lowering of the spindle (which is the motion whereby the disc is released) has lost some power in it's travel or thrust or whatever.
It's not the linkage 'arms' that have little wheels on to guide the cd out. I know this because I manually popped the cd off the spindle before ejecting it via the keyboard.
Is this common and can it be fixed I wonder?