open activities - send notification emails

open activities - send notification emails

Hello, 
When I started using CRM a couple of months ago, I was able anytime I created a new open activities to send a notification via emails. 
Since 3 weeks now, even if I tick the option 'send a notification via email' I did not receive any email. 

Would you please let me know what changed, as I really like and need this option? 

Thank you

Sabine