Heartbeat not working and machines switching offline

Hi, I've some problems with heartbeats and computers switching offline even if they're connected, lately. Heartbeats say the machines are offline, but I know that they're not; in fact, starting a scan results in a green led. After the scan, anyway, sometimes PCs come back in offline status. All of them run Windows 7. I don't know if the problem started with Inventory 16 or if it is linked with some buggy Windows 7 update, but it's frustrating, also because strange things happen if the machines switch offline during a deployment. Any suggestion? Thanks.