May not be poultry mites, perhaps it's Straw Itch Mites instead? Especially if you are not seeing anything on the Mama hen and she was nesting on the straw.
Sorry to hear you're still dealing with mites, hopefully you can source a treatment and start getting on top of it.
While Ivermectin can be helpful when treating the birds, you will still need to treat housing as mentioned earlier in the year.