Network connection drop at branch loaction after main site fortigate goes down

Hi All,

I am new to the fortigate worls, I inherited a system from a previous sys admin at my current job. We have a bunch of branch sites but when the main fortigate goes down at out corp location from say a power outage one of our sites also loses internet connectivity even though they are all hooked up and have their own internet connection obviously. I am figuring it must be some little setting in the routing of the vpn but I am just unsure of where to go hunting for this. I am comparing the tunnels and addresses/policies but almost everything seems to be configured mostly the same on the trouble site as all the rest. If anyone has any ideas that would be great and I would appreciate it. 

Thanks