This comment from PaulG.x caught my eye:

Electronics technician with 48 years in the industry here.

The common cause of the buttons losing sensitivity is that the silicone absorbs skin oils and these oils act as insulation on the pads and tracks.

If you look at the tracks under the pads that are least sensitive , you will see the oily residue. You can clean the tracks and pads with alcohol for a short term fix but the pads will exude more of the oil that is within the silicone.

A longer term fix is to soak the whole key pad sheet in Fuelite (Petroleum Spirit) Fuelite is the main ingredient in CRC Contact Cleaner (in fact it is the only ingredient). Use liquid Fuelite to do this , not Contact Cleaner because you have to immerse the silicone sheet.

Soak the sheet for 5 minutes , it will swell a little , let it dry thoroughly and it will return to normal dimension.

While the silicone has still some absorbed Fuelite in it , it will be easily torn so treat it carefully.

Then reassemble the device.

This fix should last several months depending on the state of the silicone sheet