I just lost a Buff Orp chick yesterday that was exactly as your sounds. I had marked the chick (dot of magic marker on it's head) because a week before it had an over-sized crop from eating shavings. I covered the shavings and it seemed to rally. I'm not positive that is what happened to it in the end, but it was just not eating, or drinking or running around like a chick should...oh, and it felt like a little empty husk when I picked it up, very light and never seemed to get bigger or gain any weight. (The rest of my chicks feel like little chubby chunks in comparison, and they gain growth and weight everyday!) The sick one didn't have any poop symptoms at all.
Yesterday morning before I left for work (I drive a school bus) it actually ate something and drank a little bit. By the time I got home from my morning route about 3 hours later, it was gone. Still with no poop symptoms! I'm baffled.
NewGuineaChooks is right, sometimes they just don't make it. I guess Mother Nature has a way of culling of her own.
I've also lost 3 other chicks, 1 other BO and 2 Barred Rocks that just never seemed right from the time I brought them home. They were always weak, puny and cheeping loudly all the time. I thought I could save them with TLC, but it just wasn't to be. I lost 4 chicks out of 17...not very good odds...over 25% loss... but I don't think it was my fault, I did everything according to the books (and this wonderful forum). The chicks just weren't very hardy. I just hope the other 2 BO's live and my last Barred Rock too. And I hope at least one of those BO's and the BR are pullets!
I'm pretty sure there was no disease or anything like that because all my other chicks are growing and thriving. I guess you can just get runts and chicks that aren't going to make it. It's sort of hard to accept tho...the last time I had chickens I never lost one single chick. Maybe it's just my turn.
Keep us posted on how your chick does.