Machines does not make contact with server

So, when i install the agent on a new machine, the machine pops up on the client list at the server's control panel. But, some of them does not update the "Last Inventory" section at the server. It's like no one has turned the machine on since i installed the client at the machine. I'd like to stress that this only applies to some of them, as many others works just fine. Also, if that matters, i use Windows Server and AD, but many machines are only used by one person. And i installed the agent using the local user of the machines.

If i can provide any more info, i'll be glad to help.

Thanks in advance.