My first inclination is also a magazine problem. Make sure that you try a different BRAND of magazine, I've two "cheapo" M14 mags, neither of which feed properly, but rifle works just fine with GI mags. I haven't found an AR magazine (GI) yet that didn't function properly in either of my Colt or Rock River rifles.

Otherwise, could be a number of issues, some which have been addressed already. Aside from magazine issues, the most common offender that I'veseenis a poorly assembled rifle, particularly a "kit" or "bargain" rifle. Rarely have I seen bolt hold-open problems or failures to fire from name-brand rifles.