I've built a new one. They should be able to breed right? People are telling me they need peat moss and that coco fiber will compact the stomachs of them when people on the forum bred them using cocofiber.
Plenty of hardwood leaves/wood plus cocofiber mixed with soil. How do I make them burrow?
I used this black potting mix (it was like a powder and I added water) and I mixed with the cocofiber. I mixed in a load of leaves and some fermented rotten wood along with bark. I opened the enclosure and they were sitting on the surface of it...
I say using a lot of potting soil is not the best for millipedes, as it may contain chemicals that are good for plants and not for millipedes. I use 20% cocofiber and peat, 10% garden leaf compost, and the rest is rotten wood I have shredded to a fine powder or small chunks. Make sure that the substrate is not powder, some small pieces and chunks allow for better nibbling and burrowing. In your case, just mix more wood in and place leaf litter on top of the substrate. If your millipedes don't burrow still, perhaps they are just enjoying the surface.
Top soil is probably better than potting soil. When I get in to millipedes I’m planning on using 70% topsoil and 30% coir, I’ll also mix leaves and rotting wood in. I think some coir is a good idea because it holds water and has anti mold properties to it