Yeah, it could be something in the dovetail.
The only time I've put a used plastic sight back in it seemed kind of bowed to me. I got it started then very lightly clamped it down with a small plastic clamp. The clamp will move as you tap but if you can keep it in the dovetail 'till it's started to hold it should work.
If you keep letting it pop up the edge can get rounded enough that you might have to file the plastic a bit to get a better edge. If so, don't file the side, only the upper edge.