I believe that a good relationship with your staff members start with an only-staff forum where there should be sticky thread making them know what you expect from them and allowing to ask questions or concerns they may have.
Some staff members remain inactive because they don't know until what extent their power on board are, while others simply want to help but don't know how.
In such sticky thread there must be a side note stating that whoever cannot remain active for a reason or have to go away for awhile should notify it.
Sometimes your staff could consist of students needing to prepare an exam, hence not full-time to devote to the forum for some days or weeks.
Other times your staff might have a family urgency, or job concerns that take them away, but it's their intention return.
In both cases, if you remove their staff privileges because you ignore why they are away, this may be truly frustrating for someone who wants to be good staff member, but for a reason was not active.
Now, if they are given with the opportunity to post a reason and they don't do it and simply depart, then you can proceed to demote them.
There is still a risk that they could not notify you, in example having to run to the shelter when a tornado approaches, but if they know in advance that their staff permanence depends on notify you, they will find their way to do it in a reasonable period of time.