I don't think pressure is the issue. What the last two said seems like a good culprit.
However. I must ask if you lube the bolt carrier and the upper received. If so don't for a free hundred rounds. Leave a it dry. And fire a few hundred through it. After a time it will become polished by the force of the action and feed more reliably.
At least that was my experience in Afghanistan. Deployed with a brand new colt m4. Once you fire a few hundred rounds bone dry and make sure every thing on the fire arm is peachy it will function without fail.
Another problem could be the magazine.