If it's the type with a roller-ball underneath, then it probably needs a clean. Undo the retaining plastic clip and remove the ball, clean in warm soapy water and dry throughly.
Look inside the compartment were the ball fits and you will see two, sometimes three rollers, clean these with a cocktail stick, then a cotton bud dipped in warm soapy water and squeezed almost dry. This can be done with the mouse still connected but turn off the PC first. j.