The drives do malfunction; I've had several that had to be replaced. Does yours show up here: Apple > about this Mac > more info > System Report > Disc Burning?
If it doesn't recognize any disks, I'd suggest making an appointment at your nearest Genius Bar - diagnosis is free (if you've covered by warranty, do call Apple - take advantage of your warranty).
I inserted my friend's DVD (proven to have content), saw the iMac could see no content, then went to: Apple > about this Mac > more info > System Report > Disc Burning and got the report below which says the disc is blank:
OPTIARC DVD RW AD-5680H:
Firmware Revision: 3AHB
Burn Support: Yes (Apple Shipping Drive)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -R DL, -RW, +R, +R DL, +RW
Write Strategies: CD-TAO, CD-SAO, CD-Raw, DVD-DAO
Write Speeds: 2x, 4x, 6x, 8x
Twice I inserted a commercial DVD and it was automatically ejected both times.