We had a problem where AutoCAD users in one of our remote offices could not get a network licence even though all appeared well.

The problem turned out to be a little bit too much latency on the connection between our head office (where the licence server is held) and the remote office
Users would get an error

A valid license could not be obtained by the network license manager.

Error [1.5.-15]
Error [1.5.-18]

The following fixed our problem

1. Right-click My Computer. Click Properties.
2. In the System Properties dialog box, click the Advanced tab.
3. On the Advanced tab, click Environment Variables.
4. In the Environment Variables dialog box, under System Variables, click New.
5. In the New User Variable dialog box, enter FLEXLM_TIMEOUT for the name and 1000000 for the value.
6. Click OK to close each dialog box.
7. Launch the Autodesk application.
The full Autodesk article can be found here

http://usa.autodesk.com/adsk/servlet/ps/item?siteID=123112&id=7574782&linkID=9240617