Water is gravity fed through the hoses to the nipples. To use, birds must peck at the nipple to release water. Depending on the situation and where it stops flowing, the solution can be simple. If water stops flowing to an individual drinker cup after set up or filling the tank, than the issue is often air trapped in the line. Press each plastic nipple until water flows out. If water suddenly stops flowing to an entire row or the entire system, the the issue is often debris and sediment build up overtime. Check the water filter inside the water tank, if it's clean than remove the pipes and flushing them out.