I just took my brand new Sig P225-A1 to the range today and fired 200 rounds of Federal ammo. The gun fired perfectly expect that most of the time the slide did not lock back after the last round was fired. The slide did lock back only a few times.

All of the magazines I used were Sig Sauer and brand new. Before shooting the gun I cleaned it and lubed it with Slide Glide Grease.

Does anyone have an idea of why the slide does not lock back?

What specific Federal ammo load?
What variety of Slide Glide?
What was the temperature where you were shooting?

Also:
https://sigforum.com/eve/forums...50601935/m/749101723

Federal American Eagle 9mm Luger Ammo 124 Grain Full Metal Jacket, SLide Glide Light; and it was an indoor range probably around 72 degrees.

Make sure you are not riding the slide stop under recoil. I do this frequently with my P239. My other SIGs never a problem but I bump the slide stop on my P239 now and again.

My guess is you are doing similar.

If you shoot a 1911 a lot, your thumb will naturally ride the slide catch on a SIG as you are used to resting said thumb on the safety of a 1911.
Ask me how I know this....

Yes, I do shoot 1911's a great deal and that may be the problem. I am going to check the empty magazines by pulling the slide back. I guess I have another excuse to shoot this weekend!!!

Based on the other information you provided (good on that!), I would also suspect thumb on the slide catch lever. A quick check is to fire the pistol with your left hand (good practice anyway).

Use the thumbs high technique on the last round to see if the issue persists. I agree that you may riding the slide stop.. btw, what is your opinion overall on the P225-A1? Accuracy at distance and overall handling?

I am a big fan of the 225A1. I don't like the slide stop location relative to my thumb. I ended up using a dremel to shrink the slide stop to the point that I wouldn't inadvertently hit it.

Only thing about the gun I would change. Other than it should come with XRays.

I think the gun is accurate at distance and handles very well. The stock trigger is great for a carry gun. It looks to be very well built.

I have the same problem with mine. It's my thumb.

Simple way to check is load mags with two rounds each and shoot one hand left handed if the problem goes away you know for sure it is your grip.

Had the same issue with my slide not locking back on my P225-A1 when I I got it two years ago and it went away after a few hundred rounds. Its been flawless since. Excellent pistol fits my hand perfect