Do you know whether you actually have any Hedgehogs in your garden/immediate area? Obviously they have to be present or you won't get any takers!
You could put a few leaves in the box to see if that gives an incentive. Obviously you don't want to leave the fallen leaves on the lawn too long as the grass will die underneath. Maybe rake them up + leave them in a pile in a sheltered position such as under a hedge.
Pet shops sell hay for the rabbits, how about getting some of that and putting it in your hedgehog house for added warmth that they can snuggle into. I think hedgehog houses need more than just bare walls and hedgehogs cant carry bedding, That is why they usually get under piles of leaves and twigs.
Just released you have become a new member so Hello kleftiwallah (Tony) remember you from WAB days.
The hedgehog box roof is damp except for a dry patch in one corner, this is because hedgehog is in residence, it's caused by it's body heat drying out the roof felt in that corner. There was one overwintered last year and it looks like there will be one this year too, I cleaned out last years hay and leaves in June, and in August I washed the box out with warm soapy water and rinsed thoroughly with clean warm water and left to dry out. I renewed the hay with fresh stuff, cost at the pet shop about £1.50p, put some at the entrance to the box and noticed that it was being taken into the box around end of August, I feed Hoggie as we call it every night with Spike hedgehog food, both semi moist and dry as well as some peanuts and dried mealworm in a little bowl, we see it nearly every night and sometimes two hedgehogs come in, the most I've had is three, all at the same time, it's not too late to encourage a hedgehog, just put some food out and some hay in the box and some just outside the box and see what happens. Duncan