Take the cover off, here's a screw on top of the sliding bolt plate,unscrew that until cycling becomes smooth again, .....it happens that that screw tightens up, and it takes only a few minutes to fix it , I freacked out a few times thinking my guns broke until than don't force it .
You forgot to put the retaining spring back, the one that goes on the side of the reciver behind the magazine slot, that's the one the jumps when you take the cover off if you not careful