hook_simplesamphp_auth_account_authname_alter hook is not running

Hi,

I have module setup correctly and it is authenticated fine. I have some existing drupal users and I only want them to authenticate to site. These users have 'Enable this user to leverage SAML authentication' checked. I have setting to disable auto register false. And I have utilized hook function 'hook_simplesamphp_auth_account_authname_alter' and 'hook_simplesamlphp_auth_existing_user' to map it with saml user. But none of these hook gets hit when 'Register users' option is false. Once I enable it, only hook_simplesamlphp_auth_existing_user works. Am I missing something?