[galaxy-user] Repeated Error with Tophat

Hello- I am trying to use Galaxy to investigate gene expression differences among datasets. The first time I used Tophat, everything worked fine and it returned a nice set of assembled transcripts. However, I have since tried to use Tophat on additional datasets (in the same format as the first), and have repeatedly received the following error message, "An error occurred running this job:Job output not returned by PBS: the output datasets were deleted while the job was running, the job was manually dequeued or there was a cluster error." Is this something that I am doing wrong, or a problem with the system? Thanks!