1911 Slide catch engaging mid magazine

Your thumb or bullet nose, loose plunger housing

Check for a loose plunger housing.
Check your thumb hitting the slide stop or a bullet in the magazine is engaging the slide stop.
I have a cast bullet design that was hitting the slide stop on the inside, same little spot the magazine follower is supposed to activate the slide stop.
If your sure your thumb is not hitting the release, its easy to file a bit off the slide stop.

Take the upper off the frame, put the slide stop back in, and insert a loaded or partly loaded magazine, wherever you see it happening the most.

Look for the top bullet nose hitting or at least closely approaching the slide stop lug where it protrudes into the magazine well. Look for copper tracks on the slide stop lug.

File off the color, try again. I have done it on several slide stops.

Kimber ran off a bunch of slide stops that would not engage. Then they ran off a bunch that engaged prematurely. MIM moulds are EXPENSIVE, let the customers and the warranty clerk or gunsmith sort it out.

Sometimes you need to put a dimple on the slide stop where the ss plunger makes contact otherwise it can slip off.

Oh, I just love a nice "dimple".

Putting a dimple in the slide stop is a poor solution to a non-problem. If the slide stop is made right there will be no need for dimples except on pretty girls.

Jim

I had a 1911 built by a pretty good 1911 smith and from time to time the slide stop would engage prematurely. I took it back to him and he just put the slightest dimple on the stop where the plunger held it. It's been fine for 30 years.

just stone down the slide stop where the copper marks are on it from the bullet nose pushing it up