And if you are trying to go over grout lines it will not work, must be flat and smooth. something like counter top works great. Your bathtub will hold it for you, not a solution, but it'd be in reach. I think you will need to look "outside the box" for this one. I suggest a surface that works mounted on the wall, like one beautiful floor tile, a shower mirror or maybe a radio if it has a flat surface, you could even paint and seal a board. You could also use screws to mount it permanently, seal with a bit of silicone.
My shower. Walls are smooth and hold the suction cups with no problem, with that being said, the first time I used it and I was getting out of the shower the suction cups released there hold and I fell. Caution, do not use for anything other than to hold a trowel. Could not recommend.