Hi Jessica,
~느라고 is used to explain why you couldn't to something. 그림을 그리느라고 놀지 못했다 = you were to busy drawing/painting, so you had no time to play. You wish you had more time to play, but you haven't.
~(으)려고 is used when you tell what the purpose of the action was. 그림을 그리려고 안 놀았다 = you didn't play because you wanted to draw/paint. You could choose.
뛰어가느라고 전화를 못 받았다. (Because I was running, I couldn't answer the call.) But you wouldn't say 뛰어가려고 전화를 못 받았다.