First lets look at what causes failures to cycle (not due to magazine failure).
1. gas key not properly staked
2. clogged gas port and or gas system
3. broken ejector spring or claw
4. faulty bolt (possibly cracked)
Break down the rifle and look at the bolt carrier and bolt assembly for any damage. Check springs, bolt face, firing pin etc.. Next look at the recoil dampener and spring. Has the dampener been forced out of place, or broken spring? Also check that the forward assist is not damaged in some way. Next look at your trigger group and pins. Inspect the inside of your upper for any damage or excessive wear. Be sure there are no foreign matter such as pieces of broken shell casings lodged in any part of the rifle.
In a nut shell a good breakdown of the rifle with proper inspection will reveal the problem.
Sound like the gas system is okay so look for issues with the magazine