Group not inheriting access privileges for all tables I can't figure out why tables created by one user can be accessed by a read-only user, but not tables created by another user. 

I have a viewers read-only group with this access:  

-- Grant access to existing tables
GRANT USAGE ON SCHEMA public TO viewers;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO viewers;

-- Grant access to future tables
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO viewers;
GRANT USAGE, SELECT ON ALL SEQUENCES IN SCHEMA public TO viewers;


I have two superusers in my database and one read-only user, all members of the group: 

 Role name | Attributes | Member of 
-----------+------------+-----------
 a         | Superuser  | {viewers}
 b         | Superuser  | {viewers}
 readonly  |            | {viewers}


When I create a table as user a, the readonly user can access it. 
But when I create the table as user b, the readonly user gets  permission denied for relation. 

When I check the grants, when the table is created by user a, the viewers group gets granted select automatically: 

SELECT grantee, privilege_type 
FROM information_schema.role_table_grants 
WHERE table_name='testa'
;

 grantee | privilege_type 
---------+----------------
 viewers | SELECT
 a       | INSERT
 a       | SELECT
 a       | UPDATE
 a       | ...


But if the table is created by b, the the viewers group does not have that select privilege. 

I'm sure it's simple, but I am not sure what commands will help me troubleshoot the permissions error. Where can I check to see the existing privileges and get the root of this?
The group to which the users a or b belong does not affect the permissions on the tables created by these users.

There must be default privileges defined for a, but not for b.

Check with \ddp in psql.