if the flapper is moving and the kreepy moves, that means you're getting sufficient suction through the main tube. if the pump is not pulling hard enough to pick up debris, it certainly won't be pulling hard enough to activate the flapper.

if it moves but doesn't seem to pick up, i'd guess that maybe the opening on the top of the unit, above the rubber disk that contacts the bottom, is blocked up. it will pick up very small debris from the main tube, but mostly the rubber disk disturbs the stuff on the bottom, then the suction opening on top of the disk pulls in what's been stirred up.

for what it's worth, i'm delighted with my kreepy. i used to vaccum three times a week for a half hour or more, and now the only time i ever vacuum is when we're hosting a party and i want it to be extra clean.