Simple problem with only two possible causes: (1) The slide stop spring is either broken or resting on top of the #1 pin. (2) You have inadvertently developed the bad habit of pushing upward on the slide stop each time you fire the pistol. Correct your grip; and this problem will go away.
TEST: Drop the magazine, and clear the weapon. Raise and lower the slide stop with your finger. If the slide stop snaps back down, of its own accord, after each time you raise it up then the spring is not broken, nor improperly installed. Use an empty magazine and manually rack the slide in order to assure that the slide locks open, and you haven't got a magazine follower problem. (Which, in the present case, I very much doubt.)