I wondered the same thing and called customer support. Larry returned my call within an hour. He was friendly and glad to help. He said a filtration barrier in the bottom of the pot will clog the pot. The grow medium itself acts as a filter. The pump has a filter inside the slotted half which pops off of the motor half. You should check this out also to see if the pump inlet is fully open. Mine was closed. There is a second small filter in the bubbler unit at the top as well.