You might try a brief spray of contact cleaner on the switch. I would first test where it wouldn't show to make sure the cleaner won't damage the finish. Ralph

How old is it?

Try spraying some compressed air between the buttons.

You might need to replace the micro switch. The micro switch can be remplaced by Omron D2F-T.

Probably a dodgy connection in there somewhere. Take it apart and look around...

Hi, I had the same problem with another Logitech mouse. I took apart and I realised, that the button died. I took apart a 10 years old mouse. It contained exactly the same button. I used the dissipative solder sucker and disoldered the buttons out ( I recommand to use midle or right button, because they are not use so often, so you can expect they are in better condition). Be careful! If you don't have any experience with soldering/desoldering, practise with any unuseful parts. If is your new button out, try to measure if is working properly. If so, you can solder it in. Your mouse should be ready to work.