For me, it's all the school work. I was on fire near the end of summer and I was writing a lot every single day. As school started, I began to cool off and then the homework started to come in. Needless to say, my will to continue writing shrank considerably.
Here's my current plan [tentative at best in all respects]:
The Martian Empire Trilogy
Book 1: Ruins of a Forgotten Earth [Includes Part 1]
Book 2: Colonies of the Red Planet [Includes Part 2a and 2b]
Book 3: Dystopian Dreams [Includes Part 3 and 4]