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Issue may be with furnace.Can you do some troubleshooting on furnace.
When thermostat is still calling for heat and furnace is not on, look in furnace blower compartment (most have a peep hole to look in) and see if we have an error light flashing on furnace control board.If so give e model on furnace and tell me the pattern of flash code.That will tell me what we need to concentrate on .
Ok is thermostat currently calling for heat but furnace not running ?
OK we can leave the question open until the furnace is not running but should be according to thermostat.At that point lets check for an error code. We may have a pressure switch or a limit opening