There are usually one of three reasons I'm in a slump:
1. I'm trying to force the story someplace it doesn't want to go.
Cure: To fix it, I have to back up to the last point I was excited when writing. Everything forward of that gets distilled down to its main points and completely reworked. It might mean new setting, different characters on scene, or different actions that lead to different repercussions.
2. Burn Out
Cure: Happens in every job and it's twice as likely to occur when you're self-employed. To break out of burn out, I give myself permission to not work on the story or anything remotely related to writing. Three days will usually cure me, sometimes a week is needed. Whatever it is I do during my break it needs to have a tangible end results. Something I can see and say, "Ha! I did that! I can finish a project."
3. That Other Story
Cure: This is the exact opposite of burn out. It's a case of another story stealing focus, desire, and creativity. It is the chocolate cake next to the salad. Since I have no willpower, I eat the cake. For three days. Once I've overdosed on sweet sticky goodness, I'm ready to be faithful to the main work in progress.
If the slump persists, I get a bigger bottle of wine...
What about you, dear readers? Have you suffered from productivity slumps? What is your cause and what is your cure?