The build goes out and CVS-fetches all of the libraries it uses into the "local repository", which is by default in $user_home/jbpm/repository.
Because there are so many external jars to fetch, from so many other places, in my experience, this rarely is completely successful.
Unfortunately, the checking mechanism for each "get" is to just see if a file exists that should have been gotten. Frequently the cvs get will fail half-way, resulting in half of a zip or jar file. (Yes, I have good connectivity!)
One way or another, I recommend filling a local repository till it's all correct, and then don't ever fetch remotely again. Frankly, I can't imagine how the current process is good for anyone.
Your particular problem is special... the cvs fetch for docbook support is just plain missing, I think. Go download docbook, figure out where it's supposed to go in your repository from build/build.properties (search for docbook, look at your repository to see how other directory trees are structured), and put it there yourself. Sorry, I don't remember the details of URL, etc.