If you cock the hammer and the cylinder locks , but if you keep trying to cycle it again (lightly)and it works. Sometimes when this happens you'll notice the loading gate not working properly,you have a common problem that Rugers have.
What the problem is---After tearing down the Ruger (complete dis-assembly), you will notice ,hooked to the hammer is a spring loaded plunger AKA hammer plunger. These are essential as far as cylinder timing goes. This plunger works in conjunction with the cylinder lock. As the hammer is being cocked, the hammer plunger will contact the tail of the cylinder latch. This will push the tail up and the nose of the cylinder latch will move down. This frees the cylinder so it can rotate to the next chamber. As the hammer is pulled back farther, the pawl contacts one of the ratchets on the cylinder, which starts the cylinder rotating clockwise. Soon after the cylinder begins to rotate, the hammer plunger will cam off the cylinder latch. This will allow the cylinder latch to spring up and rub on the cylinder until it finally contacts a cylinder lock notch.
When the trigger is pulled, the hammer thrusts forward and when it has traveled about 3/4 of the way, the hammer plunger will again contact the cylinder latch. This time, the hammer plunger will drive the rear of the cylinder latch down, which in turn pushes the front of the latch up and forces it deeper into the cylinder's lock notch. As the hammer plunger passes the cylinder latch, it will be pushed into the mounting hole against spring tension until the plunger has cleared the cylinder latch. It then pops back out and is ready for the next cocking cycle.
So the short version ... the hammer plunger pulls the cylinder latch down while cocking and drives the cylinder latch deeper into the lock notches when firing.
These plungers have a knack of being bound up with rust/powder residue and don't operate properly all the time..Most of the time you can soak it in a good lubricant/cleaner such as (WD40) and keep pressing on it get it to work freely and get all the crud out all of the little plunger or push the plunger in slightly, drive the keeper pin out of the hammer and clean it that way. Once you get the crud out Clean all the WD40 off. Soak with a light oil ,reassemble the gun. Rugers are a pain in the keaster to tear all the way down so if you attempt it yourself a good video is the best way to learn. I have done 3 already two were factory new but obviously sat on the shelve a while before they were sold. If this doesn't work then the plunger and/or spring is faulty which was the case on the 3rd one i repaired. Of course you can send it to the factory and they will fix it.
note
o not use WD40 or any other penetrating oil on a gun cylinder or chamber ,EVER,it can penetrate primers and casings making that defense shot not work when you need it. WD40 is not a good choice to lube anyway because after it dries it looses it lubricating factor.