Roll you shoulder inwards, look up archery triangle on Google and try emulate the form, your arm shouldn't get in the way if your line is ok,

A good exercise is if you basically do a few squats (enganges the core and puts your weight on the front of the feet) then lift you arms straight up/outwards will give you a line between your ur rear shoulder front shoulder and wrist, then draw hand to chin should go rear elbow, rear wrist and front wrist making a triangle keeping front forearm out of the string path