Hello

I got my new pond to the point where I could fire up the pump and see how my waterfall would look. It took a few attempts priming the system before the pump started to move water.
I have a Aquascape pressure filter. I switched the valve from filter to the clean setting and instantly lost prime. I had to go through the whole process of filling the primer over and over until the pump was moving water again.
My question is, why would i loose prime by just turning the valve to the clean settingon the filter? Do you have to turn it super fast?

Thanks for the help!