Try another cartidge or twenty to make sure you dont have a dud batch. If this doesn't work then the next bit may sound really silly. You may be cleaning it too much. Yes I did say that, I know because I have been guilty. If you use any form of grease as I do on moving parts then my rule is feel it but dont see it. Sparing application is the way forward. The .22 played up one night but at least you can field strip the bolt in lamplight and cure the problem. Is it ejecting etc and which cartridge is it misfiring on. The first suggests a manual cocking fault then on could be a gas charge not operating well enough.
Firstly I would try other cartridges, I have suffered recently and know others who have in the past. Even expensive caps do it, youknow the ones securicor deliver them individually. I think they are Bismuth or something like that, mind you lead is going the same way.
Good luck
James