This is a known problem with recent production Glocks, my Gen3 G27 does it while my Gen3 G23 doesn't.
You should call Glock when you get a chance and speak to one of their representatives in the Warranty Service department. They may tell you that you're limpwristing or shooting weak ammo, but that even if you're not the new ejector should fix it for you. When you ask how you can get the new ejector, they'll tell you to find a local Armorer to order and install the ejector for you. At that point you should ask for a prepaid shipping label to send the gun back to them and have them do it. If they say no, ask to speak to a supervisor or the Warranty Department manager and ask that person for a prepaid shipping label.
Once you get the gun back in Glock's hands, they'll check the fit of the extractor and replace it if necessary. They should also install the new ejector, stamped with the number "30274" for 9mm or "28926" for .40.
Whatever you do, don't pay for shipping yourself. Either insist on a prepaid shipping label or find a local Glock Armorer to install the new ejector and possibly a new extractor for you. Currently no one sells the new ejectors, the only way to get them is to send the gun back to Glock or have an Armorer order and install one for you.
I recently got a hold of a 28926 ejector to test in my Gen3 G27 and I'll be testing it today. Most people report at least better ejection with the new ejector, some people report perfect ejection, an even smaller number have reported no change at all or even worse ejection.