I suspect nutrition as ONE possibility.
Do you know the protein level on the scratch? They are often around 8%. How many chickens is the couple hand fulls being shared between? Do you free range and do you have lots of bugs/worm or only a few? These kind of things can effect your protein level. It's recommended not more than 10% of daily intake be treats, which is what most scratch is. It's like a candy bar, high in energy (calories) but low in nutrients. Birds will eat to meet their energy/caloric need, not their nutrient need. Most veggies and especially lettuce are very low in protein as well. But shouldn't be a problem if kept in balance. Peas are high in protein.
The reason I talk about protein is because of the amino acids within. This paper shows that laying hens for breeding (not eating eggs) should be fed a higher protein feed... Also, I'm not sure if we talked about your breed already but the higher protein (than 16%) especially for heavier breed like Barred Rock or Orpington is recommended. (per the previous link I posted about feed). And I will quickly note that while I don't know if it's true, both link I provided support it, I use 20% protein feed with OS on the side also for the reason of hatching more viable chicks in addition to because I always have a mix of chicks/broody's/molter/cockerel so it meets everybody's needs plus most my hens are larger breeds.
http://articles.extension.org/pages/69065/feeding-chickens-for-egg-production
One thing I wonder though...
IF eggs from higher protein feed have better nutrition for the chick to start off.. Wouldn't it stand to reason that they would have better nutrients for US to? Just thinking dangerously out loud, so don't put any stock into it.
I also do ferment my feed. It saves me about 10% but drastically changed the way the poo smelled.

And it adds probiotics while increasing nutrient absorption. Check the link in my signature if ya want. Its as simple as add feed/water/stir/wait a few days and feed.

Just sharing what has worked for me.
It's possible that bator is off gassing, but I would *think* others would have the same problem and already raised cane or reported it. And can you smell any bleach left over? That stuff seems pretty toxic.
Did your last babies make it?

(cheering you on!)