I had the same problem with my Iwata when building the Le Solei Royal. Upon inspection, I discovered that the end point, was clogged, it so happens that few weeks before I had purchased an air brush cleaning kit and in this kit came a tool that looks like a very small screwdriver were the point is very sharp, small and pointed. Using this tool I was able to remove the old paint that had sealed the end of the air brush.
This air brush cleaning kit did not cost much and I go it at Amazon, sorry I do not have the link.
Hope this help
Joe