I raced with and against a guy who was well known to let riders "dangle"; showing them an opportunity to cut in like you want/expect, then surging forward and closing the door. Enough times and that dangling rider was cooked. Remember, this is racing. If it is not to his/his team's advantage to let you in, he is under no obligation to do so. If he can wear you out, well that's one less competitor to worry about.
Ben