Does this 3rd machine show up in your device manager?

I looked up your third device and it has not returned any results even though it is receiving a new work unit every hour. My guess is that the agent is crashing or being stopped by some spyware program or something. Check to make sure DEP is not enabled, that the install directory has write permissions, your total virtual memory use is not greater than your system is configured for, etc. You might also run a virus scan and use some of the tool listed here to check your hardware memory and cpu. Did you install the agent in some non standard way? Are you using a firewall product which attempts to check stack overflows? Thanks.

Sorry I cannot credit you for results not returned. You mentioned that your problem might be due to a firewall. Which firewall product is this? Is it a software product installed on the machine or is it outside the machine? There might be a simple cure for it mentioned in one of the forum posts. You can try using the search feature using the firewall product name involved.