If you can run some code that looks like this you should be able to find it

 var collection = Membership.GetAllUsers();
            string[] roles = System.Web.Security.Roles.GetAllRoles();

            foreach (MembershipUser user in collection)
            {
                foreach (string role in roles)
                {
                    if (role.Equals(user.UserName))
                        return "same username and role name " + role;
                }
                
            }

}

That did not return any roles, the roles collection was empty.

string[] roles = System.Web.Security.Roles.GetAllRoles();  

 should return all roles defined. Therefor I quess the problem is in your RoleProvider secion of web.config.