Are these the Williams (aluminum alloy) Fire-Sights, or the old plastic ones? I replaced my plastic ones years ago with the Williams, never had a problem with them staying put, even climbing around in the Rockies of Colorado.
There are other brands. True Glo makes a Muzzle Brite TG958 set that will fit it, I have a set of those too, but are no better than the Williams Fire-Sights IMO.
I would contact Customer Service and let them know, they should replace the sight if it is defective.
You may want to check with Skinner Sights, they may have a heavy duty set to fit.
http://www.skinnersights.com/page_16.html