I'm not sure if this has been brought up or not, and it may not work for everyone, we all have our differences in body type.
I found that if I follow the instructions packet to the letter. The part about laying on one side and cocking one knee up higher than the other. I find that that position forces the abutement tab to slide off center. If I switch legs, it slides the opposite direction. Keeping my knees in alignment, and hips aligned in the same direction as my shoulders, is what puts it dead center on me.
Do anyone else notice this?