I assume they are new suction cups. I had the same issue, but I followed the recommendation of wiping them with my thumb. This helped though wondered if I was getting a little body oil on them this way. Finally I wound up wiping them off with a wet cloth with just a little bit of soap and then with the wet cloth. I use the small ones now, and it has yet to fall.
I wonder if the cups have some mold release residue on them.
PS if you are a suction cup licker, I have never seen good long term results with this unless you clean them regularly.