TestFlight invites to App Store Connect Users not arriving

during the last week, TF users for my app have not been getting invites to start testing or notifications about a new available build.
I am using the same process that I have successfully employed until now.
all testers are App Store Connect Users that are members of the organisation & all have access to this app or to all apps

when I present the TestFlight tab for this app and choose the "App Store Connect Users" group (in the Testers & Groups section):

• I see all of the users with status invited

• The build has status testing with a green checkmark (and is the only build)

but none of them got invitations. I tried "resend invite" and removing and re-adding users, but no matter what I try they don't get an invitation and can't see the app in their TestFlight app.
also this is not an issue of invalid email - I'm using my own user as well (and I just got an apple email to this address today when I uploaded the new build)

Please help.

Dave