My gas heat pump seems to work fine if the outside temp is above 55, but when below that temp, the system starts up but never blows warm air? Example, if 60 degrees before dark system works fine but then in the middle of the night and the temp has dropped, the system turns on but no heat comes out, Suggestions?

I apologize if this was sent twice but, I don't know if the first question was submitted correctly. My question is: I have a carrier unit that was installed in Sept. 2010. It worked fine in the summer but,now there is a problem with the outside unit kicking on. You can hit the reset button manually on the outside & it comes right on but it is not receiving the message from the inside to start it automatically.We just installed a new thermostat & that didn't remedy the problem. What could be causing this?