if your hen is acting depressed, it could (not saying 100% is, but it could) be caused by the lice.
to check for lice, flip your bird over and look around the vent. they like to hang out there, as that is a source of moisture for them. look like little creamy white grains of rice (and they can move quickly if they want to).
DE will not stop an infestation. if you want your bird to continue to decline, you can try DE, but infestations call for stronger measures. sevin will kill them. reapply in 10 days, as by that time the unhatched nits will have hatched, but will not have had time to reproduce on their own, yet. (according to quite a few state ag extensions' information)
i noticed it on my birds, because royal palms were looking kind of pail and just weren't thriving anymore. flipped it over, and sure enough, there were like 4-5 running away from the vent. after dusting them, their growth took off like a bolt of lightning!
it'd be nice if everything were able to be controlled by natural and organic means, but that is not always the case.
to be honest, i'm not sold on DE. i dusted all my chickens with sevin and everything was controlled. sprayed their coops and everyone got new bedding. i've also been dusting the nesting boxes of my older hens with DE. however, a couple weeks ago i was watching my brooding hen and saw a louse run to her eye and away again.
i'm not going to waste my cash on anymore DE for pest prevention. not saying it won't work for everyone, as i believe everyone is entitled to use whatever they sit fit on their own birds (well, hopefully with good judgement), but it didn't work well enough for me. i've been involved and around animal husbandry my entire stint thus far on this earth, and am able to recognize when the battle is being lost
now that they're done brooding, everyone is getting the Sevin again (she got some on the nest, but i didn't want to disturb her to badly)