You might have inserted the slide stop wrong. Slip it in a little lower than the spring and rotate up as you snap it in so that the spring end is on top of the stop. There's an illustration in the manual. Another possibility is the screw holding the slide stop spring can be too tight or too loose. Try adjusting just a bit.
Rest in peace Muggsy
"Individual Muslims may show splendid qualities, but the influence of the religion paralyses the social development of those who follow it. No stronger retrograde force exists in the world." Winston Churchill 1899