well, I'm out of ideas.  Got a RRA 16", recent build, that's giving me light primer strikes maybe 2-3 times out of a 30 rd mag.  Happens using Wally World White Box Win, Fed 62gr and some ss109.  I have tension on the trigger, about a 5 lb pull, and BANG, round ejects, fresh round feeds.  I get tension on the trigger, about a 5 lb pull and CLICK.  The hammer is definately locking back and falling when the trigger is pulled.
Nothing feels wrong when it fails.  The rounds it fails on always fire after manually cycling the bolt and loading them back into the magazine. (after inspection, of course)   The primers on the rounds that failed to fire have dents smack in the middle, but only about half as deep or as wide as fired rounds.  I've already detail cleaned the recievers, bolt, bolt carrier, firing pin and FCG and relubed with CLP--I get pretty anal about cleaning, especially when it's for malfunction isolation.  I thought it might be that I had a hammer spring that was too light, but 27-28 rounds per magazine have nice, uniform dents and no problems firing.    Any of you Gurus have suggestions?

Well, since you already cleaned it.... just check the bolt for debris real good, and inspect/compare the firing pin with a known good.

Also, ensure your hammer spring legs are installed correctly, sitting on top of the trigger pin.