Trigger button won't press down on rubber pad for PS4 controller

On my DualShock 4 CUH-ZCT1 (older model), the left trigger button won’t press down on the rubber pad somehow. I’ve tried swapping the rubber pads, replacing the rubber pad entirely, cleaning the circuit film on both up and down sides, disassemble and reassemble the controller but nothing’s working so far.

Oddly enough, it still works when I press on the rubber pad directly with my finger (without the plastic button), so I think the problem is due to the plastic pin inside the button not being able to press down far enough for the rubber pad to touch the circuit film.

This only occurs on the left trigger, while the right trigger is still working fine with no issue. Is there anything I can do to fix this and which part should I replace ? (already have a second controller but is a newer model, just wondering if I can salvage this one)