Wideth and angle of the tree from the front, bridging meaning it hits ant the front and back, but not in the midde creating pressure points. Or rocking meaning that has too much bow and only hits in the middle. Also, check the underside for anything that might be digging in, possiibly something even in the pad. TB's are usually forward, so this resistance is likely out of character for her. I also had 3 horses get Lymes disease. The initial symptoms were a total lack of forward. One was TB, one Arab, and one Appendix. All the same. Yes, I live in tick country.