New 1911's are sometimes a bit tight and the spring pressure on the plunger may be a bit strong, but most likely the cut on the back edge of the slide stop itself (the part that touches the plunger when trying to put it in) may not be cut at just the correct angle to force the plunger in and let the slide stop go in correctly. You should not have to push the "little button" (plunger) at all! The slide stop should push it in and slide past it.
This is what causes the dreaded "Idiot SCRATCH" on a lot of 1911's that you see just below the slide stop when assembled.
Now, first of all, make sure that you are not trying to assemble it with the recoil spring under pressure. The recoil spring should be loose in the gun and the slide moves freely without any pressure against it.
Line up the slot while at the same time making sure you see the hole in the barrel lug visible in the slide stop hole. Make sure you insert the round portion of the slide stop through the hole in the barrel lug. Now, lay the pistol on a flat surface (protected with a towel or whatever) and move the slide stop up towards the plunger. You can first try to line up the slide stop directly on top (side) of the plunger and using both hands and 4 fingers, push straight in on both ends of the slide stop at the same time. This should work. But if it does not, then you have to try the alternate method..........
The alternate method can cause the scratch if you are not careful. Line up the slide stop just slightly below the plunger and hold it in that position while pushing it with both hands and 4 fingers UP and IN at the same time. Keep trying. It will eventually go in and you will say, boy....that was easy.
Worst case is you may have to file off the angle of the end of the slide stop that mates with the plunger for it to have a better pushing effect against the plunger when pushing straight onto it.
Hope this helps.
