If the DE does not work, you can get various powders or sprays from the feed store that should. You will need to repeat in about 7 to 10 days. The treatment does not eliminate the eggs, so you have to get the new lice or mites that hatch out. You also need to treat the brooder.
There are different types of mites. Some live and lay eggs on the chickens. Some live in cracks and crevices around the chickens and only attack at night.
If you use DE make very sure it is food grade. The non-food grade has a crystaline form that can be very dangerous. Google silicosis, for example.
Industrial guidelines for handling food grade DE warn against breathing it. Workers are generally required to mask when working around it in powder form. From what I've seen on government sites, I think that is more that the long term effects have not been proven to be dangerous but there are some questions, so industrially they are taking extra precautions, just to be "safe". People with Asthma or other respiratory problems may be more at risk. They use a lot of weasel words like "may" instead of saying anything definite. Again, I am talking about food grade. The stuff that is not food grade has been proven to be dangerous.
I'll mention I have trouble coming up with anything that I trust on the internet about this. Government sites use a lot of weasel words and "legalese" language. Most of the other sites have their own agendas. If someone is selling it, you can tell what their agenda is. Some of the others though, I'm just not sure what their qualifications are. I will mention I have absolutely no qualifications to make my opinions valid. The main reason I am writing all this is to make sure you are using food grade, not the other stuff. The other stuff has been proven to be dangerous. How's that for a legalese disclaimer?
Food grade DE can irritate the eyes, but I have not seen aything that says it has long term harmful effects to the eyes. I am no medical expert and this does not mean there are none, just that I have not found any.
DE works two ways. It has a drying effect. It can absorb a lot of moisture. That is probably what causes eye irritation if it gets in the eyes. It can really dry out your hands if you handle it.
It is made up of the skeletons of tiny marine animals and has razor sharp edges. It kills by slicing the target mites, lice, insects, whatever, and causing them to dehydrate. Basically they lose so much body fluid they dry out and die.
Food grade DE has been approved for use on food with certain precautions, mainly about breathing it. Non-food grade DE is dangerous. Be sure you get the right stuff.