Rear Sight Issue

[Glockenheimer]

Finishing up my polymer 80 build and got the last piece of the puzzle today.....my sights. I ordered ameriglo gl-429 suppresor height sights and borrowed a friends sight pusher to install the rear. Went to put the gun in the safe and my hand brushed the rear sight and it was loose....I gave it a little nudge and it came off in my hand.....I'm no expert but I'm pretty sure thats mot supposed to happen. Any help would be greatly appreciated as to why the rear sight is loose. It seems that there is some resistance when the sight first slides in but when its centered it moves around. The slide was from 80p builder by the way. Thanks

 

[Glockenheimer]

Ended up getting a prick punch and punching the edge of the base of the sight......used the sight pusher again and seems all good.