fortimanager not registering ?

hi, 

we have recently upgraded our foritmanager, this fortimanager is already present as our asset in forticloud. When we upgraded our fortimanager, we checked it was not registered. when we enter the credentials to register it, it said the serial number is already present which means communication does not have any issue, but it is not being registered, can someone please help me to check would could be the possible reason or what to check ?