On the windshield of my car I have two suction cups, one for the camera and one for the GPS and both fall down after a while.
I try to be careful when I install them, wipe clean and moisten the plug a little and sometimes they are stuck for maybe weeks, but if the car is allowed to stand in the sun for a few hours, it is done immediately.

Any tips how to get these things to stick?

Install them when the air temp inside the car is as hot as possible.

Don't park in the sun

 

Sorry, but I had the same issue, never found a reliable solution

15 hours ago, Crossy said:

Don't park in the sun

 

Sorry, but I had the same issue, never found a reliable solution

Me too. I wonder what the reason is? Sun heats up the rubber of the suction cup, causing it to expand to the point where it loses cohesion with the windscreen?

Double sided tape  inside the suction cup  works for me

Those suction cup thing don't last long, as someone suggested double sided tape to stick it.

A very little silicone grease seems to work for me.