Hi Matthew,
If rebooting or doing the set director command is NOT helping then would advice the following to see if it helps and with the assumption that you have a DHCP server in the network. Here it goes:
Log into one AP and do a reset from GUI. Post Reset, AP will get an IP from DHCP and AP would come to standalone mode and can be managed from remote. Here you directly apply the same firmware version update to AP(keep it handy before you do this, saves time) and then after upgrade do a set director command to ZD.
Best of luck.