I got same problem with youtube, the first i block address "youtube.com", it's work, but when user use HTTPS://youtube.com, user still can access. HTTPS use SSL so i have to block SSL. But after that beside youtube, i can not access to other website with HTTPS://*** . It is my problem right now. Check guide below
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.