I'll chime in here. The fact that it laid down quickly makes me think that it is hurting big time or else would have continued to flee. Could be a bit back and caught back end of lungs or maybe a liver shot ????
Unfortunately it was jumped and sometimes that boost of "survival juices" can keep the deer running hard for many seconds, during which it can cover a lot of ground. If you do have someone with a blood trailing pooch, it definitely does help. The dog can pick up blood-filled breath spray that cannot be seen but is deposited on the ground or brush, plus the dof can scent tiny droplets of blood that can be easily overlooked. I have used a blood trailer for decades and they are extremely helpful in recovery. Good luck on your efforts to find the buck.