Internet problems in fortigate

[unknown1020]

The sdwan is configured in the fortigate. It happens that, when link A goes down, link B should take over the load but it doesn't, it has happened several times that the headquarters stops having internet. To solve this we had to configure a rule in network rules for it to work, when this should be automatic.
Do you know what my mistake could be? Well, by having the sdwan, the backup link should be automatic

[AEK]

Which FortiOS version?

When you say link A goes down you mean cable disconnected (L2) or ISP internet outage?

Are you using SLA?

Can you share your SD-WAN config?