My L3 button is not clicking any ideas how to fix

If your controller is not responsive I have found several reasons while working on these.

Sounds like your controller has been heavily used in the l3 department. The left joystick is but with a button around the axis. Under the joystick is a thin piece of metal that bends and acts like a spring over time this gathers gunk and dust and becomes sticky or the material becomes flattened from over use. Disassemble the controller and use alcohol to clean the contacts and check for a proper bend in the plate. Reassemble the controller making sure the mold fits tightly and all springs are attached