I had a similar problem two years ago where my thunderheads were shooting low and left about 4 inches at 20 yards. After several hours trying to tune to my broadheads, I finally fixed the problem. I went to stiffer spined arrows (Carbon Express Maxima Hunter 350s) and also went to a drop away rest. Not sure which fixed the problem but now my Thunderheads and field points hit pretty consistantly.

Be sure your field points and Thunderheads are the same weight but this is probably not the issue.

Good Luck