If the Zaino CLay don't work, the other clay isn't gonna do any better.
What you have could be one of two things.
The outgassing of the new plastic in your car. Or it could be w/e dressing you appied to the dash, with the heat of the sun, evaporates. Its stubborn to take off with water, a little easier with window cleaner, but this will make it a lot easier with less fuss.
http://www.autogeek.net/dp515.html
I personally grabs ome prepsol, but again, I have these things in hand.
Zaino's glass polish is the same thing as the product above, so if you have that, use it...