Ackie not digging?

Hello all- I got my first red ackie monitor about a month ago. He's 3yrs old and a pretty awesome pet so far. I've got him alone in a 4x3x4 enclosure with a foot of a sand/soil mixture in it and temps stay around 80-140 and I mist the cage every day. He's been eating crickets, dubia roaches, and an occasional chopped up hopper and eating really well. He seems healthy and happy, but in the whole time he's been here, he hasn't dug a burrow whatsoever. I figured maybe I had made a mistake not heating the soil, so I put some flexwatt tape on the cage a week ago, but still, nothing. Maybe the soil isn't hot enough? Does anyone have any suggestions of what might be better to heat the soil? I'm a little iffy about the flexwatt as is since I have a cage build out of mdf. Or has anyone had any experience with an ackie not digging before?

Let me start by saying your setup is very nice, and the soil/ sand mixture is correct. I would make sure it can keep a moisture level to it. The moisture level I go for is something that I can stick my hand down into it, pull it back out, and the hole I made will hold. In no way you do not want your soil to be soaking wet but without a good amount of moisture if he was digging around it would just fall back into its original place. Other then that its all up to your ackie onto what he wants to do. I usually see females digging more then males by the way so it may just be that reason.

how are you measuring your temps?

I've got a thermometer in the cage and the pro exotics infrared temp gun. I tried sticking my hand into the dirt and it didn't seem to hold a hole very well, so I've been misting the cage a bit more to see what kind of difference that might make. Thanks for the comments guys.

I'm betting your substrate is too cold.

Right, I was asking about that in the initial post. I put flexwatt on the side of the cage but it didnt seem to make any difference. I've been a little iffy about using flexwatt since I made the cage out of mdf wood. How would you guys recommend heating the substrate?

Also- he seems to be progressing a little since I started misting the cage more! He dug out a little dip to rest in at the back corner of the cage under his favorite hide.

Very hard to heat through MDF. Is your enclosure sitting directly on the floor? Is the floor concrete? What is the ambient temp of the room? Floor?

The enclosure is about 4 inches off the floor, the floor is carpet, the ambient temp of the room stays around 80.