it may have a bent transfer bar from shooting or dry fire .
YES, the transfer bar can catch on the firing pin and hang up if bent. Either that or take a look down into the action where the transfer bar angles into the frame to the right. It may be hanging up there. Perhaps a good lube can take care of the problem.
Really, without seeing it and cocking it, it's going to be difficult for anyone to really pinpoint the problem. It's probably best to hand it over to a QUALIFIED gunsmith or someone who really knows Ruger single actions well.