There are two possible reasons, which you can tell by launching AutoCAD. The import succeeds, since the MIC was correct, but then when the license is opened, it breaks because of environmental conditions on the new PC, for example:
1) The license has an expiration date, and the date and time on the new PC are very different from that on the exporting PC (off by a couple of days).
2) The new PC had the license previously, but the license files (C:\C_DILLA) were deleted, either directly, or indirectly by restoring a partition image (e.g. by using Norton Ghost) or reformatting the drive.
When launching AutoCAD, if it's the first case, you'll get a message asking you to correct the date and time. If it's the second case, you'll get an error 0020 and will need to reauthorize within a week or so.
If you need to reformat a hard disk or restore a partition image, you must backup and then restore the C:\C_DILLA folder, even though you've exported the license. The backup must be made after the export, and must be restored before import.
The Network and License Management Forum is specifically oriented to these issues.
--Cy--
autodesk