The pigments in spray paint cans will settle to the bottom during storage. Insufficient mixing may result in an uneven color. To help mix the paint thoroughly the manufacturers put a small metal ball in the can. Shake the can back and forth until you hear the ball rolling.
Then shake the can so the ball rolls around the bottom. Since the paint in settled on the bottom this will help mix it thoroughly. Try to shake the can for 1-2 minutes the first time you use the can. Then shade the can for 30seconds to a minute the next time the can is used during the same day.