I use Gt's and Slicks also. When I first tried them, I screwed a few on, spun them, and thought all ok was the nature of the beast[s]. Bad move, found out later I needed to spin each one.

Parallax is not your problem and has nothing to do w/ clarity of sight picture. The latter is a focus concern of the ocular lens, parallax is an optical allusion and is dealt w/ by either pre-set ranges from factory [xbow vs rifle scope] or dialed out via adjustable objective lens [AO].

While parallax can come into play a tad using a rifle scope on an xbow, it's not going to account for the shift you are experiencing

Parallax can also be defeated on two different range devices like xbows and rifles by mounting the cheek weld the same every time...........

Sorry Cossack, we gotta agree to disagree on this one.