I'm guessing your snake is in glass aquarium? I have a 40gal tank for my ball python and found that having the heating pad with the thermostat just isn't enough as the tank is so high.
I purchased a 75w ceramic heat bulb, and I keep it placed over the center of the screen. The heat lamp is connected to a thermostat, so it stays around 85, which is high enough for the ambient temps. I also keep a hand towel placed over half of the tank to keep the warm side a little warmer, and it's been working well. My temps have stayed at a constant 80-87 ambient temp, and then 90 for the heating pad, and humidity I keep around 60 as I live in the desert and find she does better with higher moisture and doesn't seem so dried out. Make sure to be checking the temps with a thermal gun and a digital thermometer/ hygrometer, as the cheap ones you get from a pet store just aren't accurate. Hope this helps