Back tension could be the answer Roy. If you pull and hold with your shoulder muscles then you are not using back tension to take the weight of the draw. Bow arm collapse will happen in this case. Happens to me when I shoot for long periods of time and my back muscles get tired.
Take a deep hook on the string, pull with your back muscles, not your shoulder muscles, and keep your back tension so that if you had someone put their fist between your shoulder blades your shoulder blades would squeeze their fist, then pull through the back tension which will cause a release from your fingers. Hold on target until the arrow hits the target.
One other answer could be you are overbowed for now. Are you new at this? What is your draw weight? How does it feel drawing the bow?