100% of all suction cups on sale are always of very good quality. They create a VACUUM in order to stick onto where we want to fix them. They're clever and very practical.
But in order to retain the VACUUM you must ensure that there is a PERFECT SEAL between the suction cup and the surface on which it is fixed!
To ensure that a suction cup stays firmly in place it must be:
Unfortunately all suction cups, even those that are extremely well fixed do not stay in place forever. This is completely normal. But there's no need to panic: in general they stay in place for between several months and one year or more. They just have to be re-fixed. The bigger they are the longer they stay in place.
A word of advice: re-fix them from time to time to refresh the vacuum.
"He who walks confidently has no fear of falling" Corneille (Poly.II,6)