I have a STI 1911 Tactical 40 SS in .45 ACP and the slide stop is hard to remove and replace when needed. This is not my first 1911 nor my only one. I mean I have even damaged the finish while taking it in and out. I know it's something simple and don't mind replacing it but need to know the problem first. I seem to think it's the spring on the slide stop plunger but not sure. thanks for the help.

With the thumb piece pointing straight down, does the stop pin move freely in the frame, with and without the barrel in place?

If so, look at the detent notch on rear face of thumb-piece, if it's just a divot, you can use a jeweler's file to put a small track from the edge of the divot to the inside edge of thumb piece, that will make the stop easier to slide past the detent. You can also look at the detent and make sure the tip is round and smooth.

I bought a NIB Rock Island. Same issue. Fit 9X19 description. No amount of lubrication helped.

I finally massaged stop in an out with a 2 oz brass hammer. I didnt remove, just rocked out of the detent and back in. After about 20 in - out cycles, was better.