Are you using a factory Ruger magazine? The feed lips on aftermarket mags tend to get out of spec and interfere with the movement of the bolt. I.e. the lips touch the bolt or permit the cartridges to bump the bottom of the bolt.
If you are using a factory ruger magazine, then it has to be the extractor or extractor spring. However, I will say this - the extractors on the AC556 are robust and don't usually break and they are not sensitive to too much/too little oil.