Some horses really do get picky about how their ankles are bent and that makes them pull or play mean games. The adice about being the boss is completely on point as far as I am concerned. If you find that your horse will give you their foot, but doesn't want you to keep it, it may be the angle. Placing it along the thigh or, better yet letting them rest it at an angle they prefer on the top of a bucket or hoof stand often helps keep them content. Some farriers carry these stands for just this reason, plus, its easier to work on a foot when you can move around some.