Disassemble the gun, remove to bolt, and use your favorite oil based lubricant. I use a product only available at a Fastenal. Works better than anything else I've ever used. Move the extractor back and forth with your fingers while lubing. When it moves freely wipe the excess oil off and reassemble and test. Remember, too much oil is worse than no oil. If that doesn't get it, find the mechanism that actuates the extractor, and check for physical damage. Look for other missing parts, like a roll pin or the like. I was missing a roll pin on my AR bolt when I bought it and it wouldn't eject casings until the dealer I bought it from gave me a new bolt.