It's not that the brand filter is not working correctly, it's that the products in your store don't all have "Adidas" set in the Brand field. Many merchants do not add brand information to their data feeds therefore making it impossible to use the Brand filter.
While this returns 2 products: http://www.ballz.com/?brand=adidas
This returns many more: http://www.ballz.com/?word=adidas
So, since you are using brand names as filters and a product is very unlikely to carry a brand name of another brand in their product title or description, you could just change this:
<li><a href="[server.url type='fullpage' query='brand=adidas']">Adidas</a></li>
<li><a href="[server.url type='fullpage' query='word=adidas']">Adidas</a></li>
That should solve the problem.