"AR Print and email selected invoices" and "Invoice Print New Invoices" sat in status running

There's two reports which are just sat in status pending. These concurrent requests are "AR Print and email selected invoices" and "Invoice Print New Invoices".

Both spawn child requests, which are in status running, but they've been sat there for a long time (in other environments they run in under a minute), which is why the parents are in status pending, but I can't see any reason why the child requests are just sat in status running.

I can see the UNIX process for CONC_REQUEST_ID, but it doesn't appear to be doing anything, and I can't see a reason for the long time to complete.