This is almost always from not waiting 15 to 20 minutes after whirlpooling to allow the convection currents (churning) to stop and the particulate matter to settle to the bottom of the pot. The other cause is not slowing the flow enough when you reach the level of the drain tube. It is vital that you use a slow drain rate to allow the wort to permeate through the hops/trub and not drag these particulates with it into the filter. Remember, this product does not work like a traditional filter. It directs the wort flow and allows the hop particulates to settle away from the drain tube and prevents them from getting suspended again and plugging the screen.