What is the indoor temp on the Nest? There is a known bug with the Nest where if the indoor temp hits the "Safety" temp, the thermostat will be locked out from calling for heat at any temperature setting above the safety temp.
The workaround is to temporarily change the safety temp until the thermostat senses it is out of that safety temp range (usually 1 degree). So, in example, if you have the default safety temp of 40, then you can just lower the safety temp to 35 and the thermostat should start calling for heat. Once the indoor temp is raised a couple of degrees, you can set the safety temp back to 40.
