New custom domain email addresses from a previously verified domain

I added a custom domain to my apple email 2 yeras ago. I added 2 custom domain emails for myself and another for my wife as a family member. 2 of the available 3 emails for me and 1 of the available 3 for my wife. Now 2 years later I want to add another custom email address for my wife. I have gone into the Custom Email Domain section of my iCloud account. In the iCloud+ Features section, in the Custom Email Domain box, my custom domain is shown. When I go into my Domain, the Manage email addresses can be seen. When I go into either my box, or my wife's box, there is no blue + sign available to add another email address.

Any idea how I add another email address? Both my wife and I should have available addreses for allocation. Interestingly the Reverify button is greyed out.