Odd. I've tried all three suggestions with no luck. A power cycle, forgetting in the amazon app and renaming the device (I renamed it back to "thermostat" after trying a couple of other names). It still won't accept my commands. If I say, "Alexa, set thermostat to 63 degrees", I get "Sorry, that command doesn't work on device 'thermostat'". If I say "Alexa, +3 to thermostat" (even though I know that's not a command it knows), it will say "I can only set the temperature between 40 and 90 degrees". If I say "Alexa, set temperature to 64 degrees", she says "which device did you want?" and then I say "Thermostat" and she says""Sorry, that command doesn't work on device 'thermostat'".
I'm beginning to think that this particular model of thermostat has very specific commands that I can issue to it. There's no AC, just heat so maybe that has something to do with it?