How is your humidity with the heat lamp? Lamps have a tendency to dry out your air.
How is your heat mat controlled? You should have a thermostat to adjust your temps so that it will not burn your snake. Heat mats also do nothing for ambient temps.
A larger enclosure is not a bad thing but not always necessary. Also you should have a day/night cycle as your snake can get stressed from constant light, remember they live in dark termite mounds