I know this is almost a decade late, but maybe it'll help someone in the future. There's a piece of rubber between the circuit board and the pushbuttons that tries to 'push' the electrical contacts of the flexible PCB onto the circuit board. Over the course of years, the rubber loses its bounce. Either replace the black rubber block (approx 3/4" x 3/8"), or I just used masking tape (~6 layers) to increase its thickness. Problem solved.
I use DS4 windows and an online joystick checker to test the controller to make sure that all the buttons were working again.