your disc tab should be grounded ONCES in its frame life to aline your disc brake caliper properly. Get a good bike shop that has the proper tools to do it tho. Infact, many placeds of the frame (headset face, and bb) area should be faced as well to give it a nice flat service for installation, best performances, and last longer. I have had the same problem myself. Your fork disc tab should be grounded too if you have front disc brake.