You probably didn't clean it enough. The Mighty Mouse has to be taken apart in order to properly clean it. You can clean the scroll wheel easily enough and sometimes that will last for awhile before it gets gummed up again.
See this LINK on how to take one apart and clean it.
Agreed with the above. There is no way to clean it without taking the mouse apart. My Mighty Mouse scroll nipple will not work anymore thanks to gunk in it I think. But I just gave up on using it. And will use my Magic Mouse instead when I get my Apple Battery6 Charger.
If you can upgrade to the Magic Mouse I'd strongly recommend it. If not just live with the nipple gumming up a lot.
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