1. Short stoke test the rifle. This is done by loading a single round into the mag, loading and firing the single round (mag left in the rifle), and confirming that the bolt locked back on the bolt catch.

2. Carrier travel test the rifle to confirm that the hammer in not catching on the firing pin collar on the forward stroke. This is done by pulling back on the charging handle, and then slowly walking the charging handle forward.  The B/C should glide forward without binding up.  If it binds, then hold the trigger bank and try again.

3. Check the upper receiver below the feed ramps for low mag feeds. If you see dents on the upper below the feed ramps, then confirm that the threads of the mag catch is flush with the outer surface of the button.  If the threads/button is flush, then as sourced with the listed posts, you may need to correct the mags by hand or rebuild them to feed/recover at the correct angle into the feed ramps.

Once you have checked the above area's, post back with the specific area in question, and we can fine detail the resolution to that specific problem area.