Try a different scope, I've noticed that scopes that worked well for me a few years ago, don't now, and it seems to be getting more so with several scopes I have. There was a discussion about this very topic on another forum, not all scopes are equally good about this for old eyes, in fact, I have a like new MuellerTacII that the original owner had the same problem, but it works fine for me, great scope btw, the equal to a Leupold VX-III in my comparisons.
Tim