Verify FortiSandbox is receiving offline updates
On FortiManager, go to System Settings > Event Log. Verify the update requests from FortiSandbox.
Network Security 
Network Security
NOC & SOC Management
Enterprise Networking
Business Communications
Zero Trust Access 
Cloud Security 
Hybrid Cloud Security
Cloud Native Protection
Web Application / API Protection
SAAS Security
Security Operations 
SOC Platform
Advanced Threat Protection
Endpoint Security