Are you using the same backing plate for both thermostats you tried? If so, that may be the problem. If not, I would check the thermostats to see if there are settings for the type of system it is set for. You will want it setup for gas furnace.
Also, make sure the thermostat is set to heat mode.
I did try using the old backing plate at first but changed it out to the new one immediately when it wouldn't work.
There is a slide switch on the back of the thermostat for "Gas or Oil" and "Electric or Heat Pump"...and it has been set to Gas.
I have the 1st stat set to Heat/Auto (fan) and the setpoints are lowered so the propane unit will kick in only if the temp in the house drops to 64.
The 2nd stat is set to Heat/Auto also but won't operate the blower even when I switch it to Fan.
And to clarify...the only way we can get the blower to work is to run the green wire to the G terminal on the 1st stat, taking the 2nd stat out of the system entirely, and turn it to Fan.
I think you asked that earlier and I didn't really answer it.