Push buttons are widely used on all kinds of electronics. After many years of use, they may become less and less responsive when you push them. Here's an example of how to fix the button on a wall-mounted AC/heater thermostat. If most buttons are not very responsive, it's better to replace the whole thermostat. Otherwise, read on. The most used buttons may be the ones for temperature UP and DOWN. So it's possible only these two are not very responsive, i.e. when you press and hold the UP or DOWN button, it takes quite some time or even never to respond to your press. To fix this, remove the batteries. Pull on the face board straight out to remove the thermostat from the wall. Unscrew all the small screws. Lift off the circuit board. Use high concentration alcohol and a cotton swab to clean the circuit on the circuit board where the UP and DOWN push buttons touch. (You may clean the other parts of the circuit as well.) Now the critical part. The rubber buttons for UP and DOWN at the bottom side of the face board may have lost the carbon coating on the bottom side that touches the circuit. Unless you can find such buttons with good coating somewhere else, you can fix this by gluing a tiny piece of aluminum foil to the bottoms of the buttons. Make sure the foil is smaller than the button and does not stretch out of the edge. Press the circuit board onto the face board, and put the batteries back. Try pressing the UP or DOWN button. They should respond to your press immediately. Put back everything and you're done.
I wish there was an easy way to re-coat the carbon coating. I tried a regular HB pencil. It made no difference. Maybe a B-type pencil would be better. But even if it is, pencil carbon may not stick to the surface for long.