Assigning someone to a role does not automatically ensure that the associated responsibilities are adhered to. People are busier than ever, so unless a responsibility has been properly automated as default behaviour, it can quickly be forgotten.
Use recurring allocations to make sure that a given individual has reserved time for a given responsibility.
The avatars in the list of responsibilities indicate which users (if any) that have recurring allocations for a given responsibility.
Click into the responsibility to see details of the associated recurring allocations.