I have ~15 workmen in my prison, and most of them seem to be standing around doing nothing. I just started a huge fence project and only 1 of them is building the fence. The rest are standing around in storage, deliveries or garbage doing nothing of use. What can I do to make them work on the fence?
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 :)
I've also seen this happen when there's a delivery log jam, which it sounds like you might have if you've just put in a huge construction order. I've seen what I believe is workmen waiting for materials to arrive for their assigned job that will never come, as the delivery area is full and the truck at the front won't move out the way until it's unloaded (which can't happen because the delivery area is full, etc).
If your delivery area is full, try either making it bigger or zone a Storage Room somewhere. This can just be a patch of ground to start with, doesn't even need to be in a building. You'll probably want to move it somewhere secure later, as it's a source of contraband.