[Aguila Blanca]

If you insert an empty magazine into the frame with the slide and barrel removed, so the slide stop move up freely? With the slide and barrel installed but the recoil spring either not installed, or left slack, does the slide stop move up freely when pushed up manually?

You're looking for what would appear to be an interference fit. The trick is to isolate where the interference is, and then file something down to relieve it.