That's a common problem with virtually all acrylics. They dry so fast that they tend to build up on the tip. I just got in the habit of wiping the tip off every time I sit the airbrush down and it doesn't bother me much anymore.
Acrylic retarders will help the problem. They won't eliminate it but they slow down the drying time of acrylics and help with it. It's available at most art supply stores under various brand names, Createx, Golden, and several others I can't remember right now. A coupe of drops in your paint cup is all it takes.