The bumpers use ordinary push-button switches, which can wear out after 100k presses or so. If it feels like you have to press it harder than normal, and the problem gets worse over time, the metal contacts inside the button may be wearing away.
It is difficult, but possible, to replace the bumper buttons. Fortunately, the button itself is a jellybean part: search for 6x6mm SPST tactile buttons, and pick a 4-pinned one with a similar appearance to the one on the controller board.
The bumper button is attached to a metal bracket with two tiny plastic pegs: slowly pry them apart. Then, you'll need to de-solder the two joints between the button's legs and the reverse side of the board. To access both pins, you'll need to move the plastic trigger assembly aside by un-clipping it from the board near the bumper button and gently but firmly pushing it up and off the pin. Do not attempt to remove the plastic trigger assembly, because it is connected to a potentiometer which is soldered onto the circuit board.
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Once you have removed the worn-out bumper button, you can slot in your new one and solder it on. I also used a dab of hot glue to attach the new button to the old one's metal bracket - if you lick your finger you can press the glue into the gap while it's soft, and hot glue can be removed if the controller lasts long enough to need another bumper button.
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You need some desoldering tools, but it's doable.