What should I do to Fix My Left Hand Shoulder Button?

My left hand shoulder button isn’t working. I have taken my GameCube controller apart before (to replace Joystick + C-Stick), but now the L.H. shoulder button isn’t working. I think most of the parts are in working order, but whenever I press down on the button, it doesn’t push down as far as the R.H. shoulder button, and makes a slight click. I am also able to wobble it slightly while pressing it down. Pictures below:

Left hand shoulder button:

Controller Inside:

Back of Controller (Left Side)