What Don says makes sense, but if you have that positive air pressure hole in the jar obstructed by something then it won't allow any air to come into the jar, or leave through the air line. The paint needs something to push, and something to pull it - in this case it's air. Without a clear path for the air to travel along it can't pick up the paint inside the jar. I too find myself having to redo the air pressure every time I change my paint even during the same paint session. Even if you have the same paint to thinner ratio you'll find that the compressor may also need to be readjusted a little. This is because of air loss while the brush is idle, and will leak tiny amounts of air especially if you have to remove it from the air line for any reason. If you have a tank like I do this makes it lose air. Also if you have any obstruction in the air line will do the same thing.
Sometimes you might have to also readjust your needle while spraying if it builds with too much paint in the tip requiring you to clean it. If your jar gets some dried paint in the line it will clog, and not spray as well. Also if your paint pickup tube inside the jar lays on the bottom with not enough room in between this too will cause it to not pick up paint properly. Sometimes this may slip if you get paint between the jar attachment point, and the inside of the tube where they meet. Try any of these things to see if that fixes the problem.
~ Cobra Chris