My guess is that you used the scope mount that comes with the gun. It is a piece of junk, I had the same problem with mine. To solve the problem short term I used a center punch to upset the metal at the front of the mount so that it would prevent the scope rings from moving forward from the recoil. What you need to do is buy a picatinny rail or even a weaver mount that is 7/8" full sized and not the tiny 3/8" .22 sized rail. You'll need to buy regular weaver rings for the scope, $7 at Walmart. When you mount the rings to the base, hold the scope forward so that the screws in the rings contact the front of the slot. Picture the rifle recoiling and with the gun moving backwards, you want the base to already to be in contact with the rings. Check out these e-bay auctions to see what I'm talking about.
http://cgi.ebay.com/UTG-RUGER-10-22-...item5ad554053a
http://cgi.ebay.com/WEAVER-PICATINNY...item2557879f18
http://cgi.ebay.com/Ruger-10-22-1022...item1c0e6020df