1. Change the entire set of extractor, spring and plunger, and the ejector and spring.
New parts are cheap, replace them all.
2. CLEAN the bolt, especially the ejector hole. Re-lube.
3. Use a chamber brush to scrub the chamber. Inspect the chamber for rings, pits, or roughness.
4. Make sure the rifle's gas system is OK. A fouled gas system can cause failures to eject. The piston should be free to move in and out about 3/8 of an inch.
NEVER lube the gas system or allow any solvent to enter it. The system MUST be run bone dry. Any lube or liquid in the system will instantly carbonize into a sticky, tar-like substance that will cause problems.
Disassemble the gas system only if you have NO other choice. When you do, use the correct wrench, and buy a NEW USGI piston and nut to replace the old ones.
When reassembling it, make sure not to cross thread the nut or damage the threads in the gas cylinder.
5. Check to make sure someone hasn't installed an "extra power" recoil spring.
6. Try a different brand/type of ammo.