"everlock" hooks won't stay up. What am I doing wrong?

I've bought about three sets of these hooks, thinking that the first (and second for that matter) set was a dud. However, on the third attempt I wonder if I'm doing something wrong. They're supposed to work by suction. You rotate a wheel to creat the suction. Am I tightening it too much or not enough. They're supposed to hold 20 kilos, be removable, and reusable. I can't deny their movability but I can deny the other claims!