Whats your forward lean setting? Are the high backs flush with your boots or is there a gap? If thats the case the the high backs might "keep hitting" you in the area beneath the calv muscle. or... you might have set an too agressive foward lean which keeps pressure on the same area.
I´m guessing the first option is the bad guy here though...crank the forward lean until the high backs top is snug with the backside of your boot.