The Windows agent can sometimes launch before the network is ready.
When the agent launches, it checks in with AWS to find the Instance-ID. This action may take place before the network is ready to receive the call so it will fail due to no value being set yet for the instance ID. If this scenario takes place, reboot the agent as the network should be ready and able to find the instance ID.
Be advised that if the instance reboots (as opposed to just the agent) it can return to the failure state. In this case, adding an agent service dependency on the tcpip service or a launch delay can solve this issue.