Hi Alan,
Please login into the ZD cli and check if you are able to ping the AP, similarly please ssh into the AP and ping the ZD and make sure there isn't any connectivity issues.
Make sure that there is route between ZD VLAN and AP VLAN.
Verify port settings on the switch.
Then, run these commands on the AP; there outputs should give some insight.
get director
Download AP support info and check last logs.
What are the changes that you do on ZD when trying to add AP on different VLAN ?
Regards,
Abilash PR.