HA Failover Issues

I have two FortiGate 300D devices running in an HA cluster.

Configuration is as follows: FGT1 # show system ha config system ha set group-id 20 set group-name "FGT" set mode a-p set password ***** set hbdev "port3" 50 "port4" 150 set session-pickup enable set ha-mgmt-status enable set ha-mgmt-interface "mgmt1" set override disable set priority 200 set monitor "port1" "port2" FGT2 # show system ha config system ha set group-id 20 set group-name "FGT" set mode a-p set password ***** set hbdev "port3" 50 "port4" 150 set session-pickup enable set ha-mgmt-status enable set ha-mgmt-interface "mgmt1" set override disable set priority 100 set monitor "port1" "port2" end When I restart the Master device the cluster fails over and FGT2 takes control. However, once FGT1 is back online, it is immediately selected as the Master device. The output for this is as follows: "FGT1 is selected as the master because it has the largest value of override priority." As you can see from the config above, I have entered the " set override disable" command so why is the device immediately taking control of the cluster again. This has only started happening today. Over the last couple of weeks, I have restarted the Master (FGT1) several times and when FGT1 has come back online FGT2 has remained as the Master device. Any ideas?

Thanks.