Look at the right side of the gun. Behind the extractor you should see a metal button that contacts the rear of the extractor. If you see a plastic button (the bearing), then the EDP (Extractor Depressor Plunger) assembly could have fallen out of the gun when the slide cover plate was being swapped, and then the EDP could have been re-installed incorrectly. If installed incorrectly, the plastic button might have more drag (friction) on the extractor than the metal end, and might cause the problem.
If you see a spring contacting the extractor, then the bearing has broken (or fallen) off the EDP and needs to be replaced, and the EDP needs to be re-installed correctly. It is also possible that the bearing fell off and the EDP was re-installed in the correct position, without the bearing, which would result in some sort of extraction problem, I would think. The only way you will know for certain that the EDP is all right is to remove the slide cover plate and the EDP and inspect the EDP.
If you are unsure about how to do this, reply to this post and I can give you instructions, but I will need a little time to make up some photos. In order to do this you'll need a 3/32" (2.5mm) punch or a Glock Armorer's tool, which happens to be a special 3/32" punch.
Update: I've got the instruction, photos and annotations ready if you need them, to show you how to remove the slide cover plate, check (or fix) the EDP, and replace the EDP and the slide cover plate.
If that wasn't it, look for a chip on the extractor tooth.
If that wasn't it, have you changed ammunition?
If that wasn't it, I am running out of ideas, but others will chime in soon.