After long periods of use a mouse can collect little particles of dirt, dust and a various assortment of goodies. Very similar to a keyboard. If you are mechanically inclined you can open the mouse and clean it. If not you can go to a local electronics store and get a can of contact cleaner and a can of compressed air. You spray the contact cleaner in and around the cracks and crevices of the mouse and imediately blow it with the air before the contact cleaner dries. While blowing with the air click the mouse to try and loosen what ever may be hindering it. I am still using the old style ps2 mouse and when it gets dirty I just rip it apart, clean it and put it back together, Been using the same one for a little over 3 years
For all intensive purposes it might be easier and cheaper to just get another mouse.