IWS flood and drain question

Hi, I am new to the forum and hope you can answer a few questions that I have about the IWS flood and drain system.

I am using a 20 pot system and all seems ok at the moment but they have just started and I have noticed that all the water is not draining out of the pots. This is not a problem at the moment but obviously as the roots get bigger it will drown them.

The big water tank keeps pumping the water into the brainpot and it keeps draining even when the cycle has finished.