iMacs' combo drive doors are stuck closed

We have ten flat-panel G4 iMacs (700 MHz) at work, and at least five of them cannot open their optical drive doors. When the eject button is pressed, I can hear what I presume to be the motor trying to push the door open, but nothing happens. The first iMac to do this had it's combo drive replaced and now works, two others never stopped working, and I didn't check the remaining two iMacs. On one that is not working, I tried opening the drive using open firmware and by holding the mouse button down while restarting (as other threads have suggested), but I could still only hear the motor trying open the door. Having 6 drive doors all fail at the roughly the same time seems like a software problem rather than a hardware problems. (Everyone does their own updates, but I think all or most are running OS 10.4.11.) Does anyone have an idea about what to try next.