Steps #8 of the VMware KB that I posted in one of my previous answers says the following:

If no packets are received, this indicates that something is blocking UDP traffic over port 902 from the ESXi/ESX host to the vCenter Server system. Inspect the physical networking environment and any software-based firewall on the vCenter Server system.

So, based you that, you will really need to check if that port is allowed on firewall.