As others have mentioned, make sure your scope rings aren't loose. Use loc-tite on the screws to keep them from moving.

Also as has been mentioned, you may have a gun that needs to be rebedded. I bought a Ruger .17 that had that problem. Sent it back to Ruger and they got it fixed. Are you shooting consistently in the same area, just not in the bull? That would suggest a bedding/receiver issue.

Good luck with it.