Russell, You baffled??.......NO! <img src=icon_smile_big.gif border=0 align=middle>
Hehe...just kiddin Mate.
One other thing to think about......not sure if it applies to you or not.
I've shot on ranges where the ground wasn't level at different yardages........say level at 20 yards......then level again at 40 or 50.
If the ground you are shooting on is uneven at your 30 yard mark.....giving a slight "Sidehill" effect on your feet its really easy to cant your bow and not even realize it.
This will force shots left or right depending on the angle of the hill.
It doesn't take much of a change.....just the slightest difference in level can go completely unnoticed if you don't look for it.
CHeck your sight's level, and shoot the bubble the same at all ranges and see if that may be it........if that's not the problem then I'm baffled too!!