I encountered the issue today of jobplans appearing hung in a "preparing" status in their queues. Upon further inspection the issue was related to an agent being offline that was being invoked in a postprocess tab of one of our jobs. The job only appeared as "waiting on host" in our queue despite having the status "preparing" when looking at the monitor of the jobplan itself.
Resolving the issue with the agent allowed the jobplan to continue unimpeded and the job itself to complete. This took a while to uncover due to the process monitoring perspective in V12 AWI not properly displaying the status of the offending job. We ended up expanding the jobplan in the queue to see that the job was listed as "waiting on host" not "preparing".
I hope this helps someone in the future.