I don't thing your F.O.C as any thing to do with it. I think its were you blade is hitting the rib not the point and sliding off to the side of the rib as it cuts it. Your F.O.C is good if you get to heavy on the point and shoot any yardage the point will coming down lower than the arrow and as it hits the weight of the arrow will not be straight behind the point and you will lose some of you energy. My arrow are at 10 % and 400 grains axis arrows and are blowing through every thing and in the ground with a 85 grain thunderheads they don't go far that way. Maybe it's the blade angle that make it slide off to the side of a rib.