I have one of those bath shelves which sticks on to the tiles with
suckers. http://images.orgill.com/200x200/3679446.jpg

The PROBLEM is that even when the shelf is lightly loaded, the suckers
lose their grip after a few hours.

I know wall-screws is a far better way to fix a bath shelf but I want a
temporary shelf.

The big supermarkets sell these sucker shelves and I tend not to think
that a large percentage of sucker shelves get returned otherwise it
would cost too much for the store just in handling all the returns.

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QUESTIONS

Why do these sucker grips shelves fail to hold.

(1) Am I doing something wrong? I clean the surface of the tile, I
let all the air out when the sucker is squashed. The sucker seems to
hold very well ... until I turn my back for a few hours.

(2) Maybe using suckers is a design so inherently bad such that no-one
here has ever much success. Or is your sucker shelf staying up?