The absolute worst thing about using propel cans is the pressure drop.
It's basic physics. The conversion of the liquid contents of the can to a gas when the valve is opened provides "air" for your airbrush. At the same time, the temperature of the liquid propellant drops. As the temperature drops, so does the pressure of the gas being released. Incidentally, your refrigerator works on the same principle.
If you airbrush for long enough, on humid days you may find ice forming on the outside of the can. Typically, with a propel can, you'd have about 10-15 minutes continuous use before the pressure drop sets in. Eventually it will drop far enough that it will refuse to pick up paint (particularly with siphon-feed models), even though there is still air flowing through the system.
Apart from switching to an alternative air source, There are only limited ways in which you can overcome this problem.
- Only use the airbrush for a few minutes at a time and allow the can to return to room temp.
- Stand the can in a container of warm (NOT hot) water until it reaches room temperature.