Yeah... I think all ranks need to be present in the sub-fleet.
For instance, if I am a Sr.Captain and the sub fleet only has a First Officer assigned, it will only show the schedules to ranks of the first officer and not to Captains or Sr Captains.
Basically, I understand all ranks need to be assigned to the sub-fleet for all schedules to be displayed. It does not compound from the lower one.
Thanks though for making me think harder!