I also have never used a fluid bed. But, that wouldn't be the issue. Like Curt said, it probably is an issue with the paint itself. The only thing to try is clean the head with Alcohol, Acetone, naptha, anything you have laying around before you dip. Also try heating it up a bit more then you would normally.
Powder painting is a tough thing to learn. Different colors act different then say plain black, or white. My Chartreuse paint is amazing to dip, along with the green pumpkin. But, when I change it over to my white powder paint, I have to heat less, and dip usually a couple of times to get a good finish. Its a pain really, but its so much easier than painting heads with an air brush, or a rattle can.
Also LurePartsOnline.com probably has pre painted heads you can buy straight from them and save the headache. Sometimes when I am in a time pinch on orders that is what I do. Shell out a few extra bucks and get a pre painted head.