First I'd make sure there isn't any build up of crud in front of the firing pin and that the firing pin moves freely second make sure the the hammer swings freely.. Years ago I used to get a number of shotguns in n that people had sprayed the action with WD-40 and it would gum up so badly it slowed everything. Same with my sewing machine, I left it at my mom's and someone used WD on it to this day if I leave it for a few months I have to re-oil it, it's getting better.

After the first shot the rest are just noise.