The employee always available may have stopped believing refusal will be received safely.

Constant availability is not always eagerness. Sometimes it is what remains after a person has learned that no gets remembered, or held against them quietly at review time. From outside it looks like commitment. From inside it can be a calculation about safety, made again every time the phone lights up.

If you manage people, notice who never declines and treat that as information rather than as reliability. Say plainly that refusal is available, then prove it once when it costs you something. If you are the one always saying yes, test the ground with something small and see what actually happens.