Reasons: 1. After use, forgotten clean spray valve; 2. Upside down or side down during storage, which causes the paint deposit to block the valve. 3. Shake can too short or didn’t shake during use. 4. Shake too hard to break the glass ball to block the valve. How to use: 1. Shake can vigorously for one minute after mixing ball rattles and shake occasionally during use; 2. After use, clean the spray valve by turning the can upside down and spray until only pressure escapes. 3. Be sure not upside down or side down during storage.