Hi all, new member and looking for some advice here.
I have a KAC 14.5 upper.
It shoots super smooth and ejects at exactly 3 o'clock unsuppressed with 68g Frontier BTHP and about 4 o'clock with 55g PMC Bronze.
The gun fires 100% of the time and can pick up the next round 100% of the time, no malfunctions ever but it will not lock back after firing the last round.
I am currently running the KAC HH buffer with SpringCo White.

Have anyone experienced similar problems before? Is this an under gas issue?

Thanks a lot

What mags are you using?  Can you manually lock the bolt in the rear position?  Factory Bolt Catch?

Pull the charging handle all the way to the rear, look in the ejection port and see where the position of the bolt face is.  Bolt face should be past the bolt catch and almost to the rear of the ejection port.

Also, the PMC Bronze is a bit lower powered compared to m193 or m855 variants.  Not sure on the Frontier.

Try a H buffer or regular carbine buffer if you have one.

Try some troubleshooting steps one-by-one to see if you can isolate the issue.  

First, is the gun wet?  Lube up the carrier, upper receiver and even the spring.

The magazines...are they all the same brand?  Try using different mags.

Next the ammo...have you tried IMI, xtac or ppu?  

If this is all checked and the issue still persists...then I'd start checking "some" gas related components.   Try using another milspec carrier (the bolt will be compatible with any milspec carrier.)

Try all of the above in tandem.  If the issue is still there, switch the spring back if you can.  I know some kac springs were a tad bit shorter than others but some not.  

Try a H buffer.  

If this doesn't do it...I'd put back all original components and contact kac.  Messing with the actual port/block may possibly void out their willingness to assist.

Also...the hh and springco should work so if you tried all the steps above that but changing the buffer fixed it...there probably is some gas leakage somewhere and I'd still reach out to kac.

Hi there, thanks for the questions and suggestions!
What mags are you using?  HEX mag
Can you manually lock the bolt in the rear position? Yes
Factory Bolt Catch? Geissele bolt catch
Pull the charging handle all the way to the rear, look in the ejection port and see where the position of the bolt face is.  Bolt face should be past the bolt catch and almost to the rear of the ejection port. Bolt face is passing bolt catch to the rear of ejection port

Thanks there, I will take a replacement mil spec carbine spring and H buffer and try it out this week

Thanks for all the hints and steps.
The gun is wet, I do have some IMI on hand I will bring to range this week and try.
I will keep everyone updated here on the issue, thanks!

Bring pmags and an extra mil-spec carrier if you have one as well.

Not a fan of non-standard bolt catches.  That is extra mass that has to be moved, and quickly, to catch the bolt for the very split second of time it has to work.  I see it in High Power Service Rifle all the time.  Guns not bolt-locking almost exclusively with the competitors who are running the spiffy extended catches.  Not every guy running one will have failure, but every failure, is a guy running one.

Push on the bolt catch to assist it in catching the bolt while firing the last round. If you catch the carrier or not at all, you have a short stroke problem. If you catch the bolt, you may other problems like the catch or over gassed.

Good Point, I will give it a try tomorrow when heading out to the range thanks alot!

Hex mags and the non standard bolt catch are red flags.  I bought a few of the hex mags a while back and had nothing but problems.  Also have tried using ambi mag catches in the past and had failures to lock back on last round.

Back from the range with some news.
All of my problems went away today for no particular reason.
I put about 100rd through it and Can't duplicate the issue.
Ran the one round lock back test with both 55g and 68g multiple times and all worked.  
Tried Pmag, Hex mag, PSA mag all worked.
Ran suppressed and unsuppressed, both no issues.

I am going to keep a close eye on it for now.
The root cause can likely be just me doing something stupid last time at the range and can't recall what that was.
Thanks for all you guys' help!