Flow Manager uses LSF to run the jobs in a flow. If a job in a flow is available to be run, it is submitted to LSF to run. LSF determines how many jobs can run and where to run them.
If your flows are all pending, I would look at LSF's status to see why they are pending. It could be your license has expired or all execution hosts are full with long running jobs from flows or there are jobs from other sources that are executing causing the flow jobs to wait their turn.