Check if Sandstorm works

Hello,

I would like to check if sandstorm works for web filtering. It is very strange becaus:

• I correctly set "Refer suspicious items to Sophos Sandstorm" under Filter Actions.
• I tried to download lot of files (exe/zip/pdf)
• I didn't see any entries in the Sandbox Activity.
• I tried to send a .exe compiled by myself, without any digital signature, and open the url in chrome, but I downloaded it successfully, without any AV scan/sandbox.

The logs entries in Web filtering has lot of sandbox="-". What does it means? Any ideas?

Thanks