[Defender Pro] Mask WP Login Failing

Please can you tell me why some of my websites the mask WP login has disapeared and its back to the old wp-login.php to access the website? I am having to go back in and add what I need to the end of the url and activate that feature again. This is becoming extra work and my clients are complaining when they cant get into their website with the secure url we have given them when we first activated the feature.

I look forward to your help

Kind Regards
Guy