The problem is most likely to be in the snap dome switch. There are several possibilities.
1. Your photo seems to show the dome shifted. It is not placed over the pcb pads in the same way that the other domes are. This has probably been this way since manufacture, and was marginally working - now has gone to marginally none working. You may find that the dome gives continuity for some direction/placement of the pressure point. Solution is to lift tape (lift as little as you can get away with) and move the dome. While you are doing this clean the pad and bottom of the dome.
2. The tape does work to seal out dirt, but it is not a gas tight seal and the adhesive itself can migrate causing problems. Lifting the tape and cleaning can resolve problems in these switches.
3. Silicon pad which is the actual button face you see on the controller may have a pedestal on the back designed to press on the dome. This may have deformed/shifted due to the dome placement mentioned in 1. The fix for 1 may also deal with this, or you might have to overcompensate, shifting the dome in the opposite direction to place it under the non deformed or shifted part of the pad.
4. This is really unlikely but the dome itselve may have failed causing it to bend in ways that don't cause contact. Replacement domes are available from many sources and some may be suitable replacements. Or you can raid the local second hand store for remotes to cannibalize.