As the plants grow they choke off the easy circulation of water in the tank. Once they fill the tank from top to bottom you may have very limited circulation over much of the tank. That means where the plants are consuming the nutrients in the water, the water gets depleted of those nutrients, and with no circulation to speak of, it takes time for nutrients to diffuse back to the depleted areas, where it is being consumed faster than it arrives. That is how I understand it, anyway. The solution is to keep the plants pruned enough that water can easily circulate and add another source of water circulation, like a powerhead.