Your mouse goes through a lot of stress taking every click, double click you make while sliding on top of a dusty surface. Just like your wrist getting Carpal Tunnel Syndrome it's a matter of time before one of its controls fail.

It is common to find the left click button fail. This can be in the form of a no-click issue or a irregular click issue. Irregular in the sense some times your clicks are not registered and some times a single click is registered as a double click.

The Fix

First of all clean up the dust, especially around the button. Start by blowing some air in to it. Ideally you can use a manual dust blower. Otherwise use your mouth. Then spray some WD40 to a cup and use a tissue to apply it to the switch. This way it doesn't soak other components. If you have the tube in your WD40 can you can use it to spray directly to the switch. But be careful not to over spray and wet other components.

If you don't have WD40, you can use rubbing alcohol. But I am afraid this will only work for a short while. I have cases where the WD40 trick outlast the mouse it self. I can't recommend any other lubricant. May be a silicone spray would work, but never tried. A friend told that using coconut oil ruined his mouse. Be warned.

If you have a case where it doesn't register a click at all, the chances are it is a mechanical failure. You can try some WD40, but it's unlikely to work.

Apple Users

If you are on a iMac, MacMini I would like to recommend you look at the Magic Trackpad. In my experience trackpads fail way less. Actually it has never happened. When I got my iMac I didn't go for a MagicMouse and instead bought a MagicTrackpad.

If you are having an issue with the MagicMouse, I am afraid I haven't had any to try it out. Sorry about that.

Chinese AliExpress / Ebay Mouse

Online stores like AliExpress and Ebay are flooding with mouse that look appealing. They also cost less, some times as much as a third. How ever appealing it may be, I would recommend you to go for a reputable brand like Logitec, Dell and HP. They are better built, last longer, and safe for your health. For example in my first 4 years of freelance life I bought 6 mouse from Ebay and AliExpress but for the last 4 years I have been using one Logitec mouse/keyboard combo.

Extra

Click Button: The click button usually is a black rectangle. If your mouse has 2 buttons there will be 2 of them. Some times this button looks an actual button. This depends on the make model of the mouse.

Open Mouse Cover: This depends on the modal. There is usually one or more screws hidden. If it is a battery powered mouse it is common to hide the scree inside the battery compartment hidden by a sticker. If it is not they are usually hidden under a sticker in the middle of the mouse or one or more of the soft plastic pads that are there to make it easy to slide. Do a Google or YouTube search to see if you can find an instructional video.