It could be that there is gunk in the firing pin channel slowing down the strike. Before spending money to have a gunsmith look at it, field strip it and thoroughly clean the bolt/firing pin assembly with an aerosol cleaner like gun scrubber or similar and then leave the firing pin and firing pin channel completely bone dry. Lightly lube the outside of the bolt but leave the firing pin and firing pin channel bone dry. Once you are done cleaning it, if you hold onto the bolt by the sides and shake it back and forth the firing pin should move freely in the channel and rattle nicely as it hits the stops at either end. Reasemble the rifle and see if that helps.