If the extractor is not pulling a chunk out of the rim of the stuck brass or bending it then you have an issue with your extractor. Make sure it is clean underneath and is not damaged. Check for proper spring pressure. It should have a lot.
Have you disassembled the bolt?
The AK is designed for the extractor to jump the rim of each round so they are usually very robust. Failure would be most likely caused by fouling.
If the extractor is clean and functioning, then the only other explaination would be for the gun to be firing out of battery (bolt not completely closed) or a slamfire. Slamfire is usually associated with the SKS but it could happen if your firing pin channel is dirty and causing the pin to stick.
Soak your bolt in mineral spirits and take a pick to the underside of the extractor.
AK-47.net: AK Troubleshooting