cause of this could be debris is lodged in the trackball.
The trackball user interface found on certain BlackBerry smartphones is similar to the trackball used in a mouse in conjunction with a desktop computer. In the event debris comes in contact with the trackball and affects the usability of the BlackBerry smartphone, rapidly roll the trackball up, down, left, or right to potentially correct the problem. If rapidly rolling the trackball does not correct the problem, please contact your wireless service provider for support options.
Please do not try to do this on your own--Bring it to your local carrier store or auth blackberry tech shop to get it repaired hope this help!
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