I had a 1911 come in the shop not too long ago, it wasn't cycling (wasn't going fully into battery) and the slide lock wasn't functioning. The slide would not go back far enough for it to engage.
The failure to go into battery issue was due to a poorly fitted compensated bushing. It was just too tight on the barrel. After I solved that is when I noted the slide lock issue.
With the barrel removed, the slide lock functioned. Add the barrel (not the bushing) the slide lock functioned. Add the bushing, the slide lock would not function. The internal shoulder of the bushing was running into the shoulder on the barrel, preventing the slide from going back far enough to engage the slide lock. I removed about .100" from the bushing shoulder and problem solved.
Moral of the story: drop in parts aren't always drop in and if you don't know what you're doing, don't.