I understand your elevation is okay. I was thinking there might be a piece of grit or a burr on the slide that's pushing the sight bar off to the left around the 30 yard mark, but it would have to be pretty substantial to move your point of impact 3" at that one spot. Try Phil's suggestion and shoot the 30 yard target with your 20 mark and see where point of impact goes. I'd also do the same thing with the 40 mark. If they're both down the middle, I'd say there is some problem with the sight itself.

I can't think of any tuning problem that would put the group on at 20, off at 30 and then back on at 40 & 50, but I can't say for sure. Arrow flight okay? How does the bow do with broadheads?