Have you ever been pestered by someone who wanted something from you? One of my students wanted to earn extra credits points to further guarantee a good passing for one of my courses, and she would not take no for an answer. I was admirative of her persistence and perhaps if she had not stopped knocking, I would have opened and granted her request.
There are too many times when I thought I was being as persistent and tenacious as possible. In foresight, if I had held on longer, it would have been opened. Whatever door you are knocking on, knock until it is open. If that door does not open, find a bigger door and keep knocking. Knock, and it shall be opened unto you!