Many times, Flow report in GMS doesn't show up. This document will help correctly assigning the flow server on the Firewall usign GMS and to show flow reports in GMS.
Assigning Flow server to Firewall/s in GMS
Pre-requisite:
Have an active Flow server in GMS deployment. server in GMS 8.x
Firewall/s must have App Visualization licensed and enabled.
See below KB for deploying / Configuring Flow server in GMS deployment
http://help.sonicwall.com/help/sw/eng/7634/8/0/0/content/Configuring_Flow_Activity.31.04.htm
To Assign Flow server on a Firewall and to see Flow report data for a Firewall in GMS follow below steps:

There are 2 modes:
Manual: Flow server will be assigned to the Firewall but the flow server IP will NOT be updated on the Firewall and need to set manually on the Firewall.
Auto: This will assign / update flow server IP on the Firewall
Note: Check below KB for Flow server configuration settings on the Firewalls