It might be good to add the external compaction id (ECID) to the table in the running compactions section of the External Compaction page and to make it one of the filters. During testing the monitor was showing that a Compactor was not making any progress (0%). Digging into the logs, the compactions were just completing quickly. Clicking on the + sign to show more information indicated that the ECID was changing. Looking in the logs confirmed this as well.