I've run into this before with both NEA and other brands of lower;
There are a couple of issues that can cause this problem:
1) Magwell being too loose
2) Bolt stop slot being too loose
3) Magwell out of position in relation to the bolt stop slot
4) Bolt stop slot out of position from the magwell
5) VERY RARELY the bolt stop finger is too short, but that virtually is never the problem.
This problem will predominantly arise when using Gen 3 Pmags, but it can happen with Gen 1/2 Pmags as well.
You will often find that steel/aluminum mags don't have this problem. I've found that PCV Beowulf mags also tend not to have the problem either.
If you compare a Pmag with a steel/aluminum mag, you may notice that the spine is thicker on Pmags than on metal mags. The plastic/polymer needs to be thicker so as not to break/crack. This means the follower is forward a skootch more than on the metal mags.
Some pmags also had an issue with flash on the follower or body pushing the follower away and the fix was to scrape the flash away with a razor blade. Again, this is not usually the issue.
Bottom line, it's a lower dimension/tolerance issue.
Aside from having the lower replaced, which comes with no guarantee that the next lower (from the same manufacturer) won't have the exact same issue, there is one reasonably easy way to solve this issue:
Buy a Colt 9mm bolt stop from Brownells and shorten the finger so that it's longer than the standard bolt stop finger but short enough to not interfere with or rub on the case heads. You also have to remove a small amount of material off the top of the bolt stop as well as grinding a bit of an angle to match the standard bolt stop. The reason a standard bolt stop has a 'ramped' top surface is so that the bolt lugs can push it down as it's cycling rearward. Otherwise the square face of the lugs would just mash into the bolt stop on the last round.
I've 'fixed' a few lowers this way and it works pretty well. If anyone wants the actual dimensions/angles for modifying the 9mm bolt stop, PM me.
....or you don't use pmags. Or you just hold the bottom of your mag when you shoot and angle the bottom forward. I like my fix better though.
