I think Danno is right on track. If you've disassembled the slide stop from the frame, you do have to be careful that you reinstall it so that the slide stop snaps down when you lift it up. If you haven't disassembled the slide stop from the frame, it could be a broken slide stop spring (although this is not one of the more commonly broken springs on the Glock). If so, be happy because this is a simple, inexpensive fix. For parts, see
www.glockmeister.com.