That will happen because of paint build-up on the tip.
The way to remedy that is to learn to always spray with air only away from the model before you begin a pass with paint. When you finish your pass with paint, repeat and push the trigger down only for air and it will blow the paint particles off of the tip. You can also soak a Q-tip in thinner and stick it into the tip of the airbrush to clean away the paint.
The Q-tips with the hollow stem are best if you can find them.
Also, remember to always push the trigger all the way down for air as that is the correct way to use an airbrush. Some people try and push the trigger down slightly for less air which is basically impossible to do.
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon