I was looking through some resources and have an article that I feel will help troubleshoot the issue you are experiencing. Its called OS X: Using and troubleshooting Back to My Mac with your iCloud account found here http://support.apple.com/kb/HT4907.
This section specifically:
Troubleshooting Back to My Mac
If Back to My Mac is not working as expected, try the troubleshooting steps below, if they apply.
- Update all your Macs to OS X Lion v10.7.3 or later (OS X Lion v10.7.4 or later is recommended).
- Choose System Preferences from the Apple () menu. Open iCloud System Preferences and check that Back to My Mac is enabled. See if any diagnostic messages are provided under the Back to My Mac section to assist you with troubleshooting and resolving any connection issue.
- Toggle Back To My Mac off and back on by deselecting and then reselecting Back To My Mac in iCloud System Preferences.
- Make sure the computer you are trying to connect to is not sleeping and is set to Wake on Demand. For more information about Wake on Demand, see this article.
- If you are using an AirPort base station, make sure your firmware is up-to-date (open AirPort Utility 6.0 or later).
- Check your network for a multiple NAT setup and reconfigure your network as needed.
- Make sure NAT-PMP or UPnP is enabled (if you are using an AirPort Extreme Gigabit Ethernet base station, clickhere for information about AirPort Extreme Gigabit Ethernet and NAT PMP).
- Ensure TCP port 5354 and UDP ports 4500 and 5353 are open on your firewall. To learn more about ports used by Apple products, click here.
All the best,