Foot itch is caused by fungi and bacteria that live on your feet. In mild cases, they generally live on top of your skin and can easily be treated with many good over-the-counter products found at your local pharmacy.
But with extreme foot itch (the kind that just won't go away) these fungi and bacteria may have penetrated a little deeper into the skin. They like to hide inside and underneath dead, calloused skin. Making it difficult for most products to reach.
Most products work by killing surface fungi and bacteria. However, remaining fungi and bacteria hiding inside and underneath dead skin may have only been weakened and not destroyed. These fungi and bacteria are left to reproduce, re-infect and come roaring back. This is what's commonly called a flare-up.