Things can go wrong - here is a number of things that go wrong and who to fix them:
Corruption at the server
This is mostly caused by a queue getting corrupted. Progzoo queues all requests so that there are only a limited number of jobs running at any one time. The queue is stored in JSON format - it has happened that the file gets corrupted. In that case you can trash the queue. This means that all waiting jobs will be lost (but when the queu is corrupt they'll never get done anyhow).
You can see the queue by visiting http://progzoo.net/r.cgi
You can trash the queue by visiting http://progzoo.net/r.cgi?trashTheQueue=0 - but change the 0 to 1.