Glad to meet you, Nicci. Welcome to BYC. Sounds like the chickens have won you over entirely - we are all in the same boat here!
You may have gotten chicks in that second batch that were not as well cared for or as healthy as the first batch. I am sorry you are having difficulty. Have you mixed the two flocks together? It's best unless the chicks are very young to have a quarantine period before you add new birds to your existing flock (periods I have seen range a lot, but it seems 2 weeks to a month is the usual range).
Here is an article on
introducing new chickens generally and another article on
introducing 4 week old chicks to older birds.
and here is a list of articles and posts about chicken ailments and treatments:
BYC Lists of Chicken Ailments and Cures
and more:
1.
Chicken ailments (an aging index to articles written over the years about chicken illnesses. Be aware that some of the links dont work and some of the authors have disappeared from BYC. The date of the index is 2012! Nothing else as comprehensive) and this updated
Chicken Illnesses Library
1 a.
Treating Sour Crop and Impacted Crop and How to Tell the Difference and
Prevention and Treatments of Crop Disorders
1 b.
Chicken Poop
1 c.
What to expect as your hen passes away from old age
1 d.
Vaccinating chicks for Mareks Disease
2.
Chicken First Aid Kit (again, an older article, but a good one; there have been a number of more recent posts discussing this
here and
here, for instance)
3.
Things I wish I knew before I got my first chick (covers chicks to old age)
4.
Maintaining a healthy coop (with lots of links)
5.
Natural healing (there are items in this article that some members may disagree with, notably the use of diatomaceous earth)