Stupid question first?
How many authorizations did you use till now? As far as I know you´ve got 5 (per account?).
PC, notebook and iPod would be 3 already for example. And if you did forget to de-authorize iTunes on an old computer before giving it away or before doing a clean install you might have reached that limit?
In that case I think you could use an option in your iTunes account to de-authorize all of your "computers". And then authorize only the equipment you now use. As far as I know that option is only available once per year.
If that isn´t your problem I can only point you to this Apple page:
http://support.apple.com/kb/TS1389?viewlocale=en_US
Just in case you hadn´t found it already.