Because the roller balls are manufactured in bulk, they can vary slightly in sizes. This margin is minimal but can result in the roller balls being a little too tight or a little too loose. If the roller balls are too tight, you can move them manually with your thumb. Once they have been moved and lubricated, you will see that they begin to move freely. If the roller balls are too loose, please contact Customer Service at 1-866-728-0070. We can issue you a replacement for any roller balls that are not working properly. Please contact Customer Service within 30 days of receipt and reference your original receipt number.