When urine is allowed to dry and remain in the carpet or other material, it begins bonding itself to the fibers. The longer it is allowed to remain the stronger the bond becomes.

Proteins (chains of amino acids) physically bond to the fibers while other portions of the urine crystallize into salts, forming ionic (electric charge) bonds. At this point it has become impossible to completely remove by cleaning alone.

In the meanwhile, mother nature begins the process of decay. This painstakingly slow process is the root of your odor problem.