Try to wash the suction cups and ruler with dish soap and cold water.
There might be some dust or grease that's preventing it from sticking.
If that doesn't work there is something wrong with those grippers.
I'd return them to the store.
Edit: Ok, I just googled and read that you're supposed to lock them.
Did you do that? And I think the little purple one is only for templates.
Here's the website.
http://www.thegypsyquilter.com/thegypsygripper.htm