Quail are apparently unlike chickens as regards their attitude towards coops, not all will use them. The whole 'dark hole' thing doesn't seem to attract them as much as it attracts chickens. But if you put feed or treats in there, they'll probably figure it out eventually.
You could try sand on the bottom. They love dust baths, so if they figure out there is sand in there, they'll probably use it more. I'm not sure how well sand bedding works with coturnix, but I use it for my buttons and I can simply clean it with a cat litter shovel when I think it needs to be cleaned. Shavings might be better in the winter though.