Brian - if it was MM Black Acrylic you did nothing wrong.. that is one of the drawbacks to acrylics but it can be overcome!!! One recommendation is to get some needle lube. It's about $5.00 for a small bottle that probably will last longer than I do... second, keep a Qtip moistened with acrylic thinner or windex nearby and wipe the tip everytime it gets to spraying wierd. (Pull back on the trigger before you wipe the tip, but DONT push the trigger down! ![Black Eye [B)]](/FSM/CS/emoticons/icon_smile_blackeye.gif)
)
The other thing that works wonders is either use Tamiyas acrylic thinner because it has a retarder built in (reduces tip dry) OR go down to Michaels or Hobby Lobby or some other art supply store and look for Acrylics Airbrush Medium or Acrylic Airbrush Drying Agent. Liquitex, Goldens and Createx all have derivatives of it and it is only about $6 for a HUGE bottle of the stuff. It slows the drying time of the paint and helps prevent tip dry. Just a few drops in every cup of paint should do it...
Hope it helps and let us know if it doesnt!
P.S. not detracting from Acrylics. I use 95% acrylics in my painting, and once you learn their idiosynchrosies they are toe to toe, if not better than enamels!