Could be that the shim under the rear mount contributed to the problem.

Never pack or shim , buy adjustable mounts instead as shimming puts all kinds of stress
on the mono tube,.

The other benefit in using adjustable scope mounts is if you use them from new you
can zero your scope on the mounts and thus keeping the scope optically centered .

The range finding ability of a scope can diminish if the optics are a long way off being
optically centered.

You can re center your optics .

Sportsmatch do great adjustable mounts.

sean.