Workflow "Running" but call stuck at "QueuedInCromwell"

Hi,
When submitting a job to the pici-firecloud/Pre-processing_b37_v3 workflow, the workflow doesn't seem to be starting even though the status is listed as running, and has been stuck at this status for over an hour.

This happened before, and was not producing any outputs, so I aborted the job so I could access the log files.
Here is a snipped of the log right before I aborted it.

2018-05-17 21:16:56,675 INFO  - WorkflowExecutionActor-ebe80853-f9d5-41be-90bb-8059e1567897 [UUID(ebe80853)]: Starting PairedEndSingleSampleWorkflow.ScatterIntervalList, PairedEndSingleSampleWorkflow.CreateSequenceGroupingTSV, PairedEndSingleSampleWorkflow.GetBwaVersion
2018-05-17 21:17:24,815 INFO  - WorkflowExecutionActor-ebe80853-f9d5-41be-90bb-8059e1567897 [UUID(ebe80853)]: Starting PairedEndSingleSampleWorkflow.CollectQualityYieldMetrics
2018-05-17 21:44:05,805 INFO  - WorkflowExecutionActor-ebe80853-f9d5-41be-90bb-8059e1567897 [UUID(ebe80853)]: Aborting workflow

SubmissionId: 9fcfc72b-3cc8-403c-b259-de505944c6d0

Any suggestions for what could be causing it to hang?
Thanks!
-Robin