Give you books send you to school and you forget to clean the airbrush thoroughly each time.
A single action AB isn't hard to pull apart and clean. Also get in the habit of using a spare jar filled with cleanser. Remove the head and wipe the pickup tube with a paper towel. Spray until the pickup tube is empty, then screw it onto the jar with the cleanser. Windex if using acrylics, lacquer thinner for enamels. Flush the system clean...the take the AB apart to clean the needle. Let dry and reassemble, ready for the next use.