Usually this is because the SAS (or cnMaestro proxy) is actually unreachable, though I have seen this happen in other situations too. Older versions of firmware seemed to get stuck in this state until the AP was rebooted though. That seems to have been fixed somewhere along the lines between 20.x and 21.x though. Whether or not the SAS is actually becoming unreachable, make sure you are running the most recent stable firmware on these radios if you aren’t already.