Two things I can think of:

- inability to actually see deer before they move. How often have you still hunted, gotten in range of deer, but not known it until they run? You need to be able to see them before they move.

- stop in the open. When you stop, stop next to a tree, does two things, lets you "blend in" and gives you something to use as a rifle rest.

If you are tracking a deer, MOVE FAST when you need to make up ground. A deer that is heading in a straight line is one you need to catch up to, going slow you won't catch up. When the track starts to wander, SLOW DOWN