Just a tip as I had a recent support request on this issue....
When you borrow a license, please ensure that your laptop is not attached to your docking station/port replicator or it will borrow the license using the MAC/Ethernet/Physical Address from the docking station/port replicator.
To check your MAC/Ethernet/Physical Address of your computer, just open a command prompt and type:
ipconfig -all
You will need to be connected directly into the laptop's network card (LAN) before borrowing a license.
I've also had this type of problem when the laptops network card is set to power down when nothing is connected. When you go to use the license while roaming off the network the network card is considered "not there" so no license is available. It can reduce battery life but I recommend *not* implementing power saving on the network card when borrowing licenses.
In addition to the two good posts above, I have also seen this fail on laptops when you disable other network devices. For example, if I disable my Bluetooth, my license will fail. This can also occur when disabling wifi cards and other devices. Be sure to test your Autodesk product while completely in a disconnected environment before you leave to be sure that the product will work off-site. Good information everybody!