Diagnosis
This is usually a static electricity or screen calibration issue, not a hardware failure.
How to Fix It
Discharge any static. Touch a metal part of the furnace or a grounded pipe. Then, firmly press and hold the center of the thermostat screen for 10 seconds. This often recalibrates the touch sensor. Release and test.
If That Didn't Work
Perform a soft reset. Go to Menu > Settings > Reset. Select Reset Schedule & Preferences. This reboots the device without losing your wiring or Wi-Fi setup. Do not select Full Reset unless you want to set up everything from scratch.
Heads Up
Do not use chemical cleaners or wet cloths on the screen. A dry microfiber cloth only. Moisture can seep in and cause permanent touch sensor failure.