It sounds to me as if your base pin might be riding forward slightly. There is a spring loaded pin in the end of the base pin that rides against the transfer bar. If the base pin slides forward, the transfer bar is not lifted enough to clear the firing pin, and hits and stops as it contacts the firing pin. Your base pin may be too short, or peened where it contacts the base pin latch. Either you will need a new base pin, or a new base pin latch.
Ruger will probably fix it free, if not, neither part is an expensive fix. Could be just some crud got into the end of the base pin.
Try this: After making sure the gun is empty, hold the muzzle up and try cocking the gun. It should cock, if it is as I describe. Then hold the gun muzzle down and try to cock the gun, it probably won't cock if its as I describe.
Does any of this make sense to you? I know what I mean, hope you do.
But then, again, it could be everything else except the base pin.