It's a little subtle hard to explain. "Didn't" would not be wrong, but it would sound a little awkward. "Wouldn't," here, emphasizes that the boss or the door did not cooperate, even though Laura tried. She asked the boss, but the boss said no. She pulled on the door, but it was locked shut. I think that a long time ago, "would" used to mean "want to" or "wanted to." So "the boss wouldn't" is a way of emphasizing that "the boss didn't want to do it," not just "the boss didn't do it." In other words, she asked the boss and the boss said no. It was not simply that the boss did not think of it. Laura is using the same idea in a more informal way also to talk about the door of the washing machine, even though it is not alive and does not have desires or preferences. It was as though she argued with the door but could not win the argument.