Daughter wanted to do a youth hunt for white tail deer.  So I got her a gun ready.  I've owned this AR for about 18 years never had any problems with it no matter what cheap ammo or mag I used.  

I bought a new free float hand guard to replace the old quad rail that was on it.  I needed to change the gas block to a low profile block.  I had one that I took off of a gun that I installed a adjustable gas block on to because it was over gassed.  I broke the punch trying to get the pin out of the original gas block for the gas tube so I bought a new one from Scheels and installed it.  I slide the bolt carrier in the receiver without the bolt with no hangups. I used my SLR gas block dimpling jig to put a dimple in the barrel to make sure that gas block was lined up with the gas port.

I also purchased a new scope that I had to sight in so I grabbed some cheap rounds and some hunting rounds to dial it in with.  Used the cheap rounds to get it close and noticed it wouldn't lock back.  Changed mags same thing.  Put hunting rounds in and same thing. It never once failed to chamber a new round.  Gun was clean before the sight in.

What would be the issue?  The low profile gas block that came from the over gassed gun, the new gas tube, or something else I'm over looking?

She did get a Fork buck on the hunt.

From your description, it sounds like it's cycling correctly, but perhaps not. Could be the bolt catch?

Where is it ejecting?
How many rounds total on the rifle?
Have you examined the BCG?
Are their signs of leaking around the gas block journal or gas tube?

Disregard, I suck at reading.

A ghetto way to check the gas block is to trombone the upper.  Cup your hand around the muzzle device, block the chamber with a rag/your hand, and flute the gas tube with your finger while you blow down the barrel.

Bolt catch looks normal no marks on it.  Manually works without any catches.

At the time I was just sighting in  the scope and didn't really look where it was ejecting about 2-3 o'clock, maybe 4.  There was lots of empty's around.
Maybe 5,000 rounds
Didn't see anything abnormal
There is a very little bit of gas where the tube goes into the block.

Did this and when I plugged the gas tube I didn't feel any air escaping.

But you don't suck in the EE!

Did you also put a new gas tube in?  I'm not 100% clear if you did from your op.  If not maybe a piece of the roll pin fell into the gas tube & jammed partially, enough to cycle a round, but not enough to push it back far enough to activate the bolt catch after the last round?

Yes I also had to get a new gas tube.  I couldn't get the pin out of the old gas block.

Change the buffer spring to one a half an inch shorter

Did you get rid of the handguard retainer and shift the gas block, which misaligned the port?

I used the SLR dimple tool to set my gas block.  it looks like it was to tight to the shoulder so I rechecked it and it was good. I used this same tool to put a adjustable gas block on a different gun and it works correctly. Attached File

I never read anything about using the adjusting screw on the adjustable gas block.

Sorry if I didn't make it clear for you guys. The adjustable gas block is on a different gun. I used the same dimpling tool on this gun to find the position for the gas block that I did on that gun.  The other gun with the adjustable gas block works perfectly.

try a different clamp on gas block and fresh gas tube.

A gun with excessive bolt speed (read: over gassed) can outrun the magazine and cause failure to lock back on empty issues with hotter ammo driving the carrier faster.

I'd try some other mags and see if that's it.

AGB's are naturally less efficient than fixed/ non-adjustable blocks, so seeing them leak and cause short stroking isn't uncommon.  Your new gas tube could be less efficient too (the bulb could be sealing poorly against the gas key).