Suction Cups Stick To The Wall Suction Cups Won't Stick To Bathtub Wall?

Suction cups won't stick to bathtub wall? - suction cups stick to the wall

I try a mirror and a basket of shampoo in the bathtub wall (it's kind of molded plastic Thye) and just keep falling down. Any proposal to encourage them to travel to? I thought I put some caulking on the suckers, but I may be able to delete it later? All advice from someone please?