New car, old cradle...

Hi,
I have a new car. I want to install the iOttie cradle in the new car.
I will buy a new sticky pad.
But... after I removed the cradle from the old car I see that the suction cup has a lot of glue on it.
So how does it work? Do I just glue it with the old suction cup? With the glue?
Looks like it won't work.
I mean... even when it was new I had a hard time with the cradle. 
The suction cup with the glue does not always hold the cradle. We have 4 cars in our family, all with iOttie cradle. In some of them the suction cup was not able to hold the cradle arm.
So how does all this work when you replce it in another car?

I wrote to customer service. They were nice enough to send me a new suction cup.
But I would like to know how all this suppose to work.
THANKS!