The problem you are having is a "failure to eject" or "failure to extract" ?? It sounds like a failure to eject. In this case it is most likely a fouled gas system; but unless you used solvent to clean the receiver and the bolt, there could be grease and carbon slowing down the bolt as well...
Products discussed: Aimpro railed foregrip; GG&G Sling attachment points.
(I'm selling them now)
A word about accessories. Try whatever you like; but really try it. Get yourself into a competition with it where you fire a few hundred rounds and see what you think. Competition either...