When you load those full 30 round mags into the gun with the bolt closed, you are pressing the first round in the magazine down, fighting the mag spring to seat the mag. It seats easier with less rounds because there is less pressure on the mag spring.

Load mags into your gun with an open bolt. The issue should go away.

"Magpul" brand "P" Mags are quality mags. "Pro Mag" magazines are not. Sometimes people get them mixed up as the names are similar.

Larry