Client notifications not working

Hello,
I am helping a client with an website and i have encountered some issues.
If i try to change the order status and check the Notify Client option, the client is not receiving this notification.
The client receive the new-order email.
Do you think you can help me with this error?

If you are after help you will need to provide more information. viewtopic.php?t=200480

Alos check both your OpenCart and PHP error logs.