There could be MANY reasons for your problem-I was a pro shooter for 20 years & have been an avid Archer/Bowhunter 45 years.
Draw your bow, anchor & then while at full draw rotate your elbow out about a quarter turn (easy to do at full draw). You will be able to shoot WITHOUT a arm guard after a little practice & you will start doing it without thinking about it after a little practice. You may want to use an arm guard hunting to hold your cloths out of the way. I don' t 7 have no problems. Good luck.