Which technique are you using for your bow arm, locked out straight, or slight natural bend? If you are locking your joint out straight, I could see there being a marked increase in fatique as this actually adds stress to the muscles and joints. Same as standing at attention with your legs locked straight vs having a slight natural bend. Same thing applies, locking out adds tension whereas slight natural flex does not.
Second would be the drawing technique. I could see where using a push/pull technique could fatigue your bow arm quicker than a different technique.
Just a couple of thoughts with the limited details you posted.
Later,
DB