I did not see any mention of the need for some thermostats to be perfectly level.
Many(such as the one shown) contain a mercury switch attached to a coiled spring that expands and contracts with changing temperature.
If they get bumped out of level they will not control to the temperature setting.
This is a reason for improper thermostat operation that I find a lot.
Stats on unconditioned interior walls and leaking return air plenums pull outdoor air down/up the wall from unconditioned spaces of the attic or crawlspace.