Those heaters need air to work. Sometimes while using them in your boots all the air gets used up. Take them out and expose to oxygen and they will start working again. Usually the best way that I have found is to open them up for fifteen minutes or so, then put them in your boots, they will be hot! and stay warm longer.
Something else I used to do while hunting was to tape one to the outside of my long johns in the small of my back. The heat would radiate into the veins in my back and make me warm all over. But be careful they get hot and will burn you if placed directly on your skin.
Good luck with all the snow and be safe.