This may be old info, but I had a job yesterday doing this very thing. Everytime I would clear a jam, load paper, unload the tray, etc. no start button. Another way I found to get the job going, assuming you have Fiery, is to 'suspend printing' for about 10 seconds at the server then click 'resume printing.' Got the job going again exactly where it left off. And considering it was variable data, that is crucial! Hope this helps!
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When the machine runs out of paper, it frequently won't show the start/print button on the panel, meaning that after loading paper you can't get it running again.
A a workaround is to press the delete job button, then cancel it. After doing this the button will show up again.