Can you hear the vault attempting to eject the discs, or does literally nothing happen when you try to eject them?
If you hear something, it is possible that the rollers that help get discs out the front slot are dirty. You may want to try manually selecting different discs one at a time to eject and see if you get different results. Discs that are very smooth and shiny (like double-sided discs) are harder for the rollers to grab (especially when dirty) whereas a disc with a rough label is easier for the rollers to grab. If you are able to get some discs out, but not others, it is time to open up the vault and clean the rollers. Make sure you clean the rollers near the optical drive if you have to go through the trouble.
The other possibility is that the picker arms that push discs up to the rollers from underneath the carousel deck are not working in that front slot location. If it helps for troubleshooting, you can remove the vault cover along with the plastic carousel cover and watch the vault in action. If when you eject a disc, the carousel moves a disc to that front slot, but the disc never lifts up toward the rollers, it is likely that the picker motor is failing for some reason.
Good luck!
Andy