I just shifted from Windows 8.1 to Ubuntu 18.04 and sometimes the left click just stop working and then I have to restart the system. I found alot of questions asking that the left click isn't working but none of them mentioned that it only occures sometimes, so may be my problem is different.
After a few updates on Windows, left click of my touchpad stopped working however on mouse it worked perfectly.
evemu-record
(it will list available HID devices, and prompt you for one to use. Choose the one with "touchpad" in its name) — does anything appears there for using the left-click?