I've had controllers do this before. A sign that it's taking to much physical wear. Usually, a controller will do that when its hardware is resting on the internal components. Or the “middle man” component isnt doing its job because it is broken/worn or out of place. The only way to know would be to take it apart. Look for an internal part that gives the trigger resistance so it isnt ever resting on the internal button after you have applied pressure. Then see if it's the trigger itself by taking off the front controller plate and seeing if the button makes contact with the necessary button. If so, check the internal buttons to see if they are stuck or in a bind. Press them a few times and see if the console receives signal. Make sure everything is secure and safely reassembled.