New G23 gen 4 issue

I purchased a new G23 Gen4 today and went directly to the range to try it out.

The first 100 rounds or so the gun operated nicely except after the final round of the magazine the slide would not lock back. The next 100 rounds (I started loading about 5 at a time only so I could get more opportunities to see what was happening) the slide would lock back only about 50% of the time after the last round was fired. There were no feed issues or ejector issues. I was shooting with a firm 2 handed grip so I don't think that was the problem. Used 2 boxes of ammo I purchased at the range, 180 gr Fmj not sure of the brand, and 2 boxes of federal 180gr purchased at Walmart.

Any thoughts of why this was happening? Does the gun need some sort of break in? Should I be overly concerned?

I normally clean my guns before the first shooting and have never had an issue like this before! I didn't clean today prior to shooting because I had the time and a convienant location to shoot. Since I have been home it has gotten a thorough cleaning. I am hopeful on the next outing it will operate a little better.

I do really like the gun, nice fit to my hand. My only other Glock is a 17 but I think this will be my new favorite.

Thanks for any input.