Tip dry is common on airbrushes especially with acrylic paints as they dry so fast by nature.
Some people use a flow agent in the paint to help keep it occuring less.
I just get in the habit of constantly picking the paint off the needle tip with my index and thumb nails as I shoot air through the airbrush, and then go back to painting. Others use a Q-tip soaked in the appropriate thinner and push it into the tip of the needle and clean it that way.
The Q-tip's with the hollow plastic stem work best for that if you can find them.
I don't know if Polly S paints are very good at being airbrushed as I have only heard people liking them for brush painting mainly. I have yet to try them through an airbrush so I can't speak from experience.
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon