FME Server will sometimes queue jobs and not run them even though nothing is running

Hi,

running FME Server 2020.0.2.1,  Win32 (as connects to smallworld DS)

have two engines

no job router, just uses the default queue. both engines have been assigned to Default Queue

the default queue has priority 5

It will run 80% of the time but sometimes falls down. It may be falling down when more than two jobs are sent to the server at the same time but can't confirm this yet...

many thanks