Membership : new WP users not automatically added


I have a backoffice script that adds a user to a subscription plan (using the Membership plugin) after payment is complete.

I have configured what plan a user should be added to after registration.

This works fine for me in a couple of membership sites. New WP users are automatically added to that default.

In a new membership site I'm building, it doesn't work though. Is this a bug or is there another configuration I'm overlooking?

Thanks for letting me know,