I'm sorry your chicks are having a rough time of it. I am confused as to why the hatchery would recommend Terramycin for the babies if they are just chilled. Providing the right temps would be the answer to that problem, imo. Besides, if chilling were to kill them, it would happen well before 10-14 days of age.
What kind of chicks are they? Are they meat birds?
The fact that the shipped chicks and your hatched chicks appear to be dying of the same thing after almost 2 weeks of being in the brooder- I would investigate their brooder, surroundings, and food source.
Maybe the water source has high mineral or chlorine content as compared to when you brooded other chicks in the past?
Are they eating anything other than chick starter? Is it a fresh bag of feed? If they do get treats, do they have access to grit? They could be getting something stuck in their crop.
How is the poo? Does it appear to be normal?
Are the chicks chirping loudly or excessively? Do they seem to be in distress?
I don't know much about diseases, but it also seems you may have a lingering disease from a previous ill chick that is slowly infecting the others. Have you tried removing the chicks to a new brooder, or completely disinfecting the one you have, along with all feeders, waterers, etc.?
Were your shipped chicks vaccinated for Mareks and Cocci?
I'm just grasping at straws here- I don't have much experience with chick deaths, although I do have one of my own suffering a very strange illness right now.
Good Luck with the rest of the babies- keep us posted.