What color paint? White paint is notorious for volcanoes and difficult to get 'settled down'. Several people have noted that white (maybe because it's heavier?) is more problematic than any other color. If not that, then probably the paint has gotten damp. Dry it in a pan in the oven on low heat. Also, try a brown paper bag and seal it on the end of the pipe; I (and lots of folks..) get better results than with a coffee filter. Your mileage may vary. Hope this helps.