The drain tube on mine is on the bottom, not the top. And I've turned that knob all the way around and it made no difference.
I think I may have figured it out, but don't trust it. I moved it further up in the tank so the pump is like an inch or so below the surface. This seems to have fixed it, but I don't trust it because there's nothing to hold it where it is in the collar and if it falls for some reason, the cup is going to overflow.