It’s a user caused issue. You took the frame apart and installed the slide stop incorrectly. Lucky for you, its an easy fix.
You need to remove the slide stop and reinstall. This time make sure the spring on the slide stop is under the locking block pin. Give it the "flick test" before installing the slide. Flick the slide stop upward. It should spring down on its own. Insert an empty magazine and you should see the slide stop move upward as the mag locks in place. Remove the mag. If the slide stop stays up.... reinstall again.
Here is a good book to have lying around if you ever want to verify what you are doing: http://www.lonewolfdist.com/Detail.a...D=186&TERM=boo