This isn't uncommon with the .45 frames.
Once the slide stop lever's plunger butts up against the edge of the side plate, you can use your thumbnail to depress the plunger to let the slide stop go the rest of the way into its normal position against the frame. Also, make sure the top of the slide stop lever is below the bottom edge of the slide (which it doesn't appear to be in your pictures).
Usually folks forget to align the rear of the slide & frame notches, or else don't remember to push the barrel back just a bit before inserting the slide stop lever (or tip the muzzle up to let gravity drop the barrel back so the barrel lug is out of the way of the slide stop lever's pin as its inserted).