Mechanical mice almost always need cleaning. The easiest way to clean them is to remove the mouse ball to access the X and Y rollers. A careful inspection of the small rollers usually reveals gunk.

This stuff can be removed using a Q-Tip dampened with rubbing alcohol and a steel dental pick or toothpick to help free the particles. The ball can also be cleaned with an alcohol-dampened rag.

You may also want to use the compressed air to blow the remaining particles out