I have this problem all of the time. My muse decides to work on something she isn't suppose to be, or she keeps coming up with new plot bunnies all of the time, but refuses to finish ones. One thing that I've found that helps is reading material related to what I'm suppose to be working on. Either parts of the story you have finished, or stories similar to yours, or stories with at least the same characters seem to work. That way your muse will be flooded in the right direction. Good luck.