HA issue

Hello Dears

I have two firepower devices (HA active/standby) connected to two fortigate firewalls (HA active/standby) when both of the fortigate firewalls are working the HA status become faild while when i stop one of the fortigate devices the HA status become fine anyone have idea about this issue ? take in mind the connection is :

FG1-Port3<-->FP1-Port3

FG1-Port4<-->FP2-port4

FG2-Port3<-->FP1-Port4

FG2-Port4<-->FP2-Port3