JDE/Peoplesoft provide a good white paper on this topic. A couple of things to check is if the subsystem is running or is cluttered up with jobs. You can check this out by going into submitted jobs, but stop on the screen where you are prompted to select the server. In the Exit Bar you should have an option for subsystem jobs.
You must submit your subsystem UBE once to the server to turn on the subsystem. (The job always remains processing so it must have a separate job queue.) Then, as you process transactions which call the subsystem UBE, the jobs should appear in the subsytem.
However, the subystem can become backed up by not stopping the subsytem prior to a reboot. This must be done in order to keep the files clean. I manually purged the file using SQL to delete all of the bad subsystem records when ours quit working. Hope some of this helps. Heather