Then the circle has important feedback, which is that no one is able or willing to fill the role. On a process level, if that happens, do a round on where you see the underlying roots of the issue and then address that. For example, everyone might be stretched too thin or the role description is unclear and people shy away from it. Or no one has the necessary expertise and the circle has to support people in building the necessary skills or acquire new members with those skills.
For more information, check out our handbook Many Voices One Song section 3.6