Thermostat won't always start heater on its own
You need to preform a test that will tell you if the thermostat is bad or is the problem in the furnace.  If you can find a time when the t-stst is calling for heat and the furnace is not responding, at the furnace check to see if you have 24-volts on the white lead coming back from the t-stat.  If you do, the problem is in the furnace.  If not, the problem is in the thermostat.  These thermostats are mass produced with an emphasis on mass part.  They do fail.