fortigate not able to register on fortimanager

Hi Team,

I am Not able to register fortigate 80c on fortimanager VM64.

when i re-register the device and re-login to fortigate it is showing me its managed via fortimanager.

but i am not getting any unregistered device on fortimanager to be registered

there are other devices already working fine in fortimanager.

fortimanager firmware 5.6

fortigate firmware 5.4.4

could you please give me some troubleshooting steps.