strange, I can make mine stick to a wooden table top, even. When it's stuck to the windshield I sometimes have a problem removing it, I'm afraid I'll break something it sticks that well. I'll ask the obvious, you are pressing the center button of the suction cup and then flipping the lever over, right ? I forgot to do that the first time I used it, it did stick but wasn't steady. Only other things I can think would be warped rubber cup, really dirty glass or a signigicant change in temperature - if you mounted it when it was very cold then it became much warmer like sun on a black car, defroster on, heated windshield... I don't know that would make enough of a difference to loosen the grip though.
I'm a brand new user though, hopefully someone else might be more help.
Check the surface of your suction cup and make sure the plastic is still smooth and not worn around the edges. Then, like mentioned ^^ make sure there are no severe temperature gradients between your mount and the obbect you wish to mount it on. Check and ensure both your mount and mounting area are clean. Once you've positioned you're mount where you want it to be, press the button and push down as hard as you can the lift up the clasp. You could also wet-mount it. Set the suction cup in warm water then mount while it's still warm