I've asked a professor who I did research with over the summer to write me a letter a recommendation for graduate programs, but he hasn't responded for over a week. I've resent him an email and have received no answer, but a peer who also worked in his lab over summer got an almost immediate response when asking for a letter. We have both done the same research in his lab and known him for the same amount of time. I don't mind if he declines, but I haven't received any word from him. Should I just move on and ask another professor?