It stops working because the trick depends on the credibility of your resolutions; if they lose their credibility they stop working. Breakdown of Will explains why your resolutions need credibility, but in short, at the moment the countdown finishes, your reversed preferences mean that staying in bed seems to carry a higher payoff than getting up, but sticking to the resolution still seems to have a higher payoff than abandoning it because of all the future mornings it will work for. If the resolution doesn't have credibility, you can't believe in those future mornings.