Why arent all of my workmen working?
I've also seen this when there is a job "stuck" in the queue, such as needing to move an item but the workman cannot find a path to it.
Check your job list for failed jobs and try to clear them.
Sometimes they bug out and think they are stuck. Sometimes removing doors or manually making doors permanantly open lets them suddenly spring to life and proceed. In most cases annoyingly i had to fire the ones stuck and re-employ new ones to do the project I just undertook :)