This is a very common problem with the G502 and the plastic part hitting the switch is constructed in a way that it blocks the movement before hitting the switch too deep. So the best way to fix this is to replace the switch like @isakrittl said/linked.
A cheaper way is to switch the switches. For me my left button is word out and need the replacement while the other keys are still good.
There are 5 Omron D2FC-F-7N build in, so what I do is to change the G9 switch to the left mouse button, as I rarely use it.
While you do this make sure there is no dirt under the buttons. I had to disassemble all the top parts to clean them. You have to do this carefully and I was missing any instructions for that. Sorry for the bad language as I'm no native English speaker. And good luck to all with the repairs!
Also a hint: The transparent plastic over the laser is not very firm attached - I would suggest to fix it with for example hot glue. (don't put glue in the line of the laser!) I've had this with the G502, G500s and even the very durable MX518. As the result of the loose part your cursor line will wobble a little bit while you move - resulting in a horrible inaccuracy.