Coyotes can be a very tough animal to kill at times. As often as not, you are having to shoot them while they are moving. For that reason, you often hit them too far back in the rib cage.
Also, Quartering shots (coming and going) means that you need some pretty good penetration to get to the vitals. You often have to shoot through shoulders and hips on quartering shots.
A couple of bullets really do a great job on these tougher jobs, the 55g Sierra BTHP and the 55g Sierra Sp(flat base or Boat Tail).
Also, when predator calling, over the years you may have the surprise of calling in a bear or a Lion, nice to NOT have a plastic tip bullet for these surprises.