I work with licensing every day and the following simple rules help me, and my clients, out of problems every time.
There are a few things that can cause problems with the license file. One is the email you receive from Autodesk Licensing. I've received files that when wordwrap is off it is one long contineous line. Another time I received it in a Microsoft Excel spreadsheet. There have been other issues too. Sometimes the issue is when we copy/paste information, it inserts characters that can cause issues too.
If you are on subscription your license file will contain a "Package" section, this *must* be copied, or appended, into your license file when you combine them. However the instructions in the email from Autodesk say to copy from the "INCREMENT" line on down. If you do not capture the Package section then your license will not work.
Another thing that can cause problems within the license file is the format it is saved in. You must use Notepad and even then you may still have problems. Tab characters within the license file can cause problems and if the lines do not terminate with a hard return or if it has extra spaces at the end it can cause the license not to work. I've even received a license file from Autodesk in an Excel document. Copy/Paste from email or one file to another may paste in some undesirable information. Sometimes the file loses all the returns and is all on one line (when viewed with the word wrap turned off).
1) Always upgrade the license manager to the latest version shipped by Autodesk (for 2011 that is the 11.7 version).
2) Copy the entire contents from the license file you receive into a new notepad document and save it. Re-open the file and see if it still looks right. If you need to append this to your existing file, then remove the old information, you can copy paste from here. Then copy/paste the next license into this license file.
3) Remove any leading blank spaces or tab characters.
4) Ensure you are saving the file in ASCII file format.
5) Ensure the server name and the MAC address are the same as what is shown at the top of the existing license file. We know the existing license file works, if the MAC is different in the new one it won't work. All the encoding, all those 4 character groupings, are encoded using the MAC and if the MAC changes, it invalidates the entire license file.
6) Once you get your new license file in place, stop the service and configure for the new license file, then start the service. Since you stopped/started the service there is no need to reread the license file.
7) I leave the existing license file alone and make a copy of it, then paste the new information into the copy. Give it an appropriate name, keep it short. Then when I stop and configure the service I pick the new license file. This way I have a good backup copy (the original license) to go back to if things are not working in the new license file.
Also, it is your responsibility to ensure your license file has the correct number of licenses per your license agreement with Autodesk. I know in our busy day we just click "I accept" all too often, but you really do need to Read it.