Fortimanager add device probe faile

Hi I have Fortigate in HA mode. I'm trying to add it in fortimanager using discovery method by fgt root vdom management ip. But the Add device wizard in fgfm adom is giving me error "probe fail" while adding device. Does anyone have faced such issue? what could be the reason of probe fail and how to overcome it? I have checked communication between fortimanager to Fortigate, it's able to ping vice versa and all traffic is allowed. Is there anything I'm missing out as prerequisite to add device in fgfm.