Is the magazine failing to lock into the receiver when the capacity is full, or when it is downloaded by a few rounds?  Most magazines, to include Pmags, when full to maximum capacity, will not seat and lock into the receiver if the bolt carrier assembly is locked into battery, as the top round in the magazine sticks up too far and cannot recess enough to allow room to lock the magazine into the magazine catch.  HK 30rd magazines, will actually hold 33rds in the 30rd magazine, as HK allows for the extra room in the magazine so the follower have enough clearance so you can seat and lock it at the 30rd  maximum capacity.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Yep, mag is fully loaded when you can get the top round at least 2/3 of the way of almost allowing another round to the stack.


As for the Magpul mags, the P  mags did have a swelling problem that could cause problems with insertion and free drop, while the E mag solved these problems instead.
[youtube]https://youtu.be/UwKSTQVppOY[/youtube]

To clarify, no generation of PMAG has ever had a "swelling" problem but the EMAG (as well as the PMAG GEN M3) was optimized to accommodate the tighter HK416 magwell, and will therefore also fit within a wider range of AR15 tolerances.