I've used straw for my feral's house but don't think it's very warm. It's on the sunny side of my house which is usually the leeward side, within the fenced yard, and I replace the straw when it gets too matted down. House is used, for the straw is shaped into a cat bed within a week of my setting it up - already, this fall.
OP, if you think the cat(s) are not using their house, check and see if the wind goes in. Your picture sure looks sheltered, but perhaps the wind goes around that little wall and funnels right in or something.
Cats like flowerpots. They're round, draft proof, and often just the right size.
One might almost consider a flower pot inside the cat house!
Reading the cathouse thread (from that link) makes me want to get reflective insulated styrofoam. Don't know how well it will work in the shelter I have which is the house that came with this feral when he was TNR to my house. I'm sorry that thread got closed, I would love to know how she cut that stuff and - what? glued it in place?