Couple tips - scuff the pieces lightly with sandpaper or an emory board before gluing instead of washing them and possibly leaving soap residue. This allows for better mechanical bond and freshly scuffed plastic is "clean" by nature.
Also, ensure you're clamping the material (in this case ABS, though mounting my male snaps to nylon web that conforms to the shape of the armor better is my preferred method), not the the actual metal snaps themselves. I've seen the metal react with glue and warp, and you really want the ABS to be what's glued in.
Keep at it!