A fairly common problem with the 1911.
First thing to check is for bullet contact with the engagement lobe of the slide stop. Look for copper marks on the inner corner of the lobe. Removing a little bit of metal with a fine cut file usually solve the problem. Take one or two swipes then check for contact. Repeat until done.
Sometimes other things can cause that issue. Tolerance stack, out of spec parts, etc. The slide stop plunger spring might be weak, the slide stop plunger may not protrude far enough. The plunger tube may be crimped/improperly reamed/loose on the frame. Check against print specs and repair/replace as needed.
I sometimes end up putting a dimple about .005"-.010" deep in the back face of the slide stop thumb piece where the slide stop plunger tip rides. I put it at the point that the plunger touches with the slide stop seated in the down position. Just deep enough to provide a little extra resistance to the slide stop.
Sometimes the slide sop itself is out of spec and needs some additional fitting or replacement with a better part. Note that you should expect to have to fit every single part of a 1911. Assume that nothing is drop in fit and you will avoid that particular disappointment.
