For me, it's lack of focused time. I know I need to write more, but I tend to focus on the short marketing articles instead of the longer book. I work well in spurts of an hour - or in 8 hours at a time. It's hard to put together that 8 hours, so I can easily crank out 1-2 short articles in the hour; whereas in my mind I need the longer stretch to work on a book. Of course, now that I write it through, that's just a way of thinking that can be changed. So gee, you may have helped me "break through" that idea just by asking! Thanks!
My working habits have changed a lot lately. I used to write in long chunks of time, but that's just not possible, now. I have forced myself to get a lot done in 30-minute to 45-minute chunks.
I've wanted to write those articles for over 6 months, but it wasn't until I vowed to get them done in short chunks that they actually got finished!
More important, I encourage you to think in terms of writing less, but recycling more. Specifically, Roy Peter Clark, of the Poynter Center, has just written a great book, 50 Essential Strategies for Writers, that was based on a series of--you guessed it--50 blog posts. http://www.amazon.com/Writing-Tools-Essential-Strategies-Writer/dp/...
Think of your blog posts as "chunks" that can be assembled into chapters of a book, and you'll get your book done in no time at all!
Doesn't procrastination come with being a writer? I am always working on at least eight or nine books at a time, but not necessarily mine. My books seem to take a back seat to other authors, probably because I don't set a disciplined deadline for myself. I have two projects right now I want to finish writing by the end of the summer.