i cant say i know much about mice, but i use to have that problem, i posted on a forum of the company that made my mouse and they said that most optical mice have a piece of clear film over the red light thingy.
recieving this i took my mouse apart, carefully, and remover the piece of film.
my mouse now works perfectly, i looked at the film/plastic under a microscope (no im not a scientist, my sister has 1,lol) and there was a slight scratch on the plastic, maybe this is the same for you,
if you do decide to do this, i suggest you be very careful there are many fragile parts in a optical mouse, well there was in mine, and there small so if the fall out its a git to get em back in,lol.
hope it works out.
MilkyBar