First thing, disassemble and thoroughly clean your Magazines. The Follower may not be rising far enough to actuate the Slide Stop Lever.
Second thing, inspect the Slide Stop Lever extension to see if it can be activated by the Magazine Follower Shelf. If broken, replace.
With an EMPTY/unloaded Pistol and clean EMPTY Magazine, insert the Magazine into the Pistol and pull the Slide fully to the rear. Release the Slide. If the Slide is held back, your problem is not the Pistol and may be your Ammunition is not powerful enough to full travel the slide back. If the Slide is not held back, you may have a problem with either the Magazine or the Slide Stop.