As stated, you have a timing problem. While unloaded, slowly cock the hammer just until it "clicks in" and stays in the cocked position. Note the position of the cylinder, then pull the hammer back fully, until it stops. Bet 'cha money the cylinder will rotate until it is indexed correctly. A dangerous situation. Send it back to Ruger and they will fix it for free.
Unfortunately, this is more common on Ruger SA than we think. I had seen this on several Rugers we tested, couple chambers at least. When shooting Ruger SA, make sure that you pull hammer ALL THE WAY BACK, until hits grip frame.
If you have a chance, try Freedom Arms, you will see what is proper timing. I had chance to get a good deal on one; field grade, 7.5 barrel, 454 and 45 LC cylinders, premier grips and scope, and was stupid enough to let it go. What a mistake...