I made some gum paste flowers and I can't get the petal dust to stick, it just brushes right off. I'm using pastry grade brushes and the petal dust I bought at my local cake supply place (may have been Wiltons but I'm not positive). Any suggestions?

Petal dust for flowers should have a chalk like consistency - extremely fine powder (like talc or cornstarch kind of). It should stick when brushed onto a dry flower. If however - there is gum glue on the spot you are trying to dust - the color won't stick there.
I don't believe that Wilton has any actual petal dusts in their product line.
In a pinch you can use Artists Non toxic chalk pastels. They come in a lot of colors and work wonderfully on flowers. I use them routinely.

I am not sure what you mean by pastry grade brushes but I just use artists paint brushes (bought specifically for dusting flowers).

I hope I am helping but since I am not sure what the texture or type of dust you have is it is difficult to think of any thing else. : )