We're really sorry for missing out on the opportunity to help you. Hope you were able to resolve the issue with the Closed notifications .
If the option is turned on and the ticket status is set as Closed, it should definitely trigger the email to the user. The only instance where the email is not sent is when the helpdesk account is not verified. If that's the case, we would not allow outgoing emails as a part of our spam policy unless the account is verified.
It can also be a case of the emails being bounced out at the recipient's end. In case if you're facing email delivery issues, it is suggested to send across the details ( email address of the requester,ticket sample,etc ) to email@example.com .