good advice from tree climber.i have done the same thing and it works well.i would take the scope back and get another one rather than shiming it.it depends on the cost of the scope.normally.scopes under $100 are the ones that have given me this kind of problem.the ones i have had this problem with were trades or hand me downs[they knew the scope did not have enough adjustment to zero and did not trust it]most manufacturers will let you ship the scope back for inspection and repair.the only problem is with low cost scopes,the postage and bother sometimes is hardly worth the trouble.if you have high mounts or see throughs,could cause the problem also.good luck.