I don’t know really anything about Glocks but it seems there is some issues with there mags and the finicky AR9.

So I finally got a chance to fire off my Spikes ar9 that is complete spikes set up to use Glock mags. After 7 rounds and sometimes 5 it jams up. Failure to feed.  I was using Glock 19 10 rd mags and the guy nest to me just happen to be firing off his pc9 using Glock 17 mags. I switched and it seemed to run perfectly,  no feed issues.

So my question is, is this a known issue?  Is it a spring tension issue or a follower issue?  Is there any way to upgrade my mags or should I just grab some Glock 17’s?  Sad part is these same mags run flawlessly in my River PC9.

Any advice would be great so I don’t waste useless money throwing at guesses.

What does the feed cone on the barrel look like?  A lot of AR-9's use ancient feed cone geometry and often get rounds hitting the extension and not chambering.  Mad-Machinist can recut the feed cone, it helps solve that issue.

I am having a similar problem, I have a Spike's tactical lower and  PSA 10" complete upper.  Works perfectly including LRBHO but fails to load the last round.

I used both Magpul 15,21,27 round magazines and factory Glock 19 magazines

The LRBHO does work when it does not jam 2 out of 9 times the rest jam on the last round in the mag.

1 I found the problem when I tried to sight in, I would load a mag with 1 round, hit the bolt release and it pushes the round forward into the mag well, crushed the bullet back inside the case, and jams the bolt with the case perpendicular to mag well.  
2 If I load three or more round and hit the bolt release feeds find until the last round where only 2/9 times it will feed properly on the last round.  However when it does feed the last round the LRBHO does catch.
3 7 out of 9 time when I load up a mag the last round will smash/jam.
4 A few times the LRBHO caught with the last round still in the mag, when I hit the bolt release it smashes/jams the round

Now I was running an extended 9mm PCC buffer which was longer and heavier, and I switched to a standard 9mm PCC buffer but no positive effect.

I was using this for NRA ARC rifle matches, and performs perfectly but all stages I hit this jam causing me points/penalties for misses and procedurals.