go to the chaosgroup forums. there are way more threads on this there but...what version of vray are you running? are you sending a job to distribute render on backburner? you should try simple tests with no GI and large buckets to see what's going on with the render nodes from the workstation with the vray license dongle. if the servers are set to restatt on render end give it like 30secs before hitting render again!
Don't try calculating the LC on all of them. they're all different speeds and some might not even finish calculating. again too many variables. visit chaosgroup forums