Late reply, but just got back from a business trip overseas. Your issue is a result of the buffer retainer hole being too far rearward. When the upper is closed on the lower the back of the BCG is supposed to push the buffer off of the retaining pin. When the firearm cycles with the upper on, the buffer should never touch the buffer retainer. You can visually inspect this two ways, the first is to separate the upper and lower. You should be able to see the buffer protruding beyond the rear of the receiver (towards the fire control area) where the receiver extension threads into. The second way is to look for the rear of the bcg pushing the buffer off of the buffer retainer when closing the upper on the lower. A light helps with this. If you really want to try fixing this yourself you'll need to try to extend the buffer retainer hole more towards the fire control pocket and then use a shim to keep the buffer retainer towards the front of the receiver. The hole is 1/4". I've seen other reports of this issue on POF lowers before (and that's with matched POF uppers too).