Your'e undoubtedly not thinning the paint enough. And you probably gummed up the inside with inadequate cleaning...here's what to do....disassemble the airbrush(needle & tip) and clean off any minute trace off dried paint with TESTOR'S liquid cement, which works great for that; then soak your cup in a bath of the same. BE CAREFUL to put the liquid glue in a suitable vessel that's not gonna get eaten through. AND CLEAN YOUR AIRBRUSH THOROUGHLY after each use!