You may want to try "foiling-in" the bottom of the tank so that more heat is trapped there. And this has gone back and forth for a while on here, but I like to put the probe of the thermometer between the UTH and the bottom of the enclosure. To me that seems to give a more accurate reading, plus you DEFINITELY don't want to run the risk of your snake accidentally moving the probe if it's in the tank with him/her.

I just happened to think, are there any vents around the tank? A cool breeze from an A/C vent would make keeping temps up a bit difficult. You may also have to close your vents off a little in your room to allow the room to be warmer. I see you're from Charlotte, what are your avg. temps in winter there? Do you have a tough time keeping the house warm? That may also be something to look into as well.