I have the 14.5" and it has / had issues as well...by issues I mean it was more like a bolt action. Fire, misfire, fire, misfire...Almost all light strikes and the bolt not fully closing.
I have managed to improve it and get it to cycle more reliably...to the point where I can empty a 25 round mag with only one or two FTF's. Still have one small trouble spot to fix and I think I can get it to 99/100.
If your issues are similar, it's probably the following...
Extractor has really limited movement and a very tight spring. As a result, the bolt was tilting slightly (there seems to be a fair bit of play in the bolt's lateral movement), and rubbing on the receiver and was not completely going into battery so the round wasn't seating right...Filed a hair off of the rear of the extractor to relief it a bit, now bolt closes completely every cycle. I can post some pictures later to show what I mean. This helped but wasn't enough...
Check your bolt assembly. On the recoil spring guide, I noticed some wear marks (actual chips and gouges) inside the channel where the hammer comes up to strike the firing pin. Long story short, I dremeled that channel to allow the hammer to move freely without contacting the guide. This helped a lot...I am now at about 90% but I am still seeing faint rub marks on the guide / hammer. Very faint, but there still is some contact. Don't want to remove any more material for fear of damaging the guide so still have one mod to go.
For the finale, I am going to install a stronger hammer spring to counter any loss of power in the friction with the recoil spring guide. Combined with the natural "polishing" which will occur, and given the improvement so far, I am confident this will do the trick.
Hopefully this helps. I considered just selling the upper and buying something else, but figured I'd just be handing my problems off to someone else...now that it's working, I'm glad I didn't.