I agree with all the above....you are not focusing when you need to. It's 90% mental and 10% physical.
A good drill for elk hunting:
Run a mile and then sprint the last 100 yds.....grab your bow.....shoot 5 consecutive arrows into your deer 3-D at whatever distance you choose. If you can't keep the shots in the kill zone then you're gonna have a tough time when Mr Bull shows up screaming in your face.
That's as close as you're going to feel to the real thing as you can possibly get without being there. Bowhunting elk is day after day of hours upon hours of hard work and boredom punctuated by a few seconds of shear terror and excitement.
You're gonna have to get a handle on the focus issue
Make it a daily routine....to pick a spot (the heart) on everything you see; the kill zone on your neighbors dog, your kids basketball, your wifes car.....whatever, just pick a spot and focus on it til thats all you see....nothing else. Make it a habit
Brett