Welcome to the Forum!
Some hogs won’t use a hog house until it has become ‘part of the furniture’, so it isn’t surprising the hogs haven’t used it before now. But, the hog house will have poo in it because it’s been used for feeding. Hogs very often poo around feeding areas, especially if there is more than one hog around. If you want to continue using the box for feeding, it is best to clean out the poo and wash it with boiling water to kill any parasite eggs.
If you want it to be used for nesting, don’t use it for food. Hogs will occasionally decide to use feed boxes to nest in, but it isn’t ideal. Food can attract predators to the area.
So if you want the hog house to be used for nesting, once it is cleaned with boiling water and thoroughly dried, put in a handful of nesting material and leave more nearby. For a hibernation nest hogs use a huge amount of material, so make sure there is lots of material around. Medium sized leaves and long grasses are favoured materials. But most hogs prefer to do the interior decorations themselves – and are far better at building nests than we would be!
Maybe you need another house/box for feeding? It is best not to have feeding and nesting houses too close together so as not to attract predators to any potential nest.
Good luck. Hope someone takes up residence. Some of the males will start hibernating soon.