Symptoms
All of the engines are running large jobs and you have published a large workspace to the FME Server. The FME Server logs start reporting that no engines are running:
Cause
Resolution
On the Server Core the max JVM heap memory for FME Server 2011 and lower is 128MB. If you change the max JVM heap memory to 512MB then Server Core should handle it. In FME 2012 and higher the default max JVM heap memory is set to 512MB.