Some mites only come out at night and feed on the birds while they are roosting. During the day they hide in the nooks and crannies of the coop. That's why it's so important to treat both the birds and the coop at the same time, to get rid of them all. If you take a bird off the roosts after dark and check with a flashlight, you may see them then, look under wings and around the vent in particular. You an also wrap a piece of sticky tape around the roosts, at the ends, with the sticky side out, and check in the morning, see what you catch in the sticky tape.
Lice and mites will often find a bird that is sick or weak from something else, so not uncommon to find them infested. And they can weaken a bird a lot, make them anemic, so your birds will be a lot happier and healthier if you can get rid of them.