Root Cause of Red Down Arrow in FortiManager

I have a Red Down Arrow indicator on two of the Fortigates in our fleet of about 30 Fortigates.  In Device Groups, there is a red down arrow beside two of the Fortigates and when I drill down a bit deeper, I also see a down arrow beside the internal management IP Address, but I can still ping the mgmt address.  The end users at the remote location have not reported any problems since this morning.  Note that we rebooted the firewall at the remote site.

VPN connections are also up and running between the remote site and our primary and backup sites.

Hope you can help me determine the root cause of the down arrow. 

Thank You

Bill