I know that this is going to sound unbelievable.. but it sounds like dehydration!
Whats the water situation like? Bottle or crock?
Babies get dehydrated VERY easily, even if it wasn't summer.
If you have 4 or more kits to a cage for grow out, they need to have more than 2 water bottles, and possibly a crock of water. If anything less.. the kits will stuff their faces under the nozzle and nobody gets enough.
Separate the fat from the skinnies.. keep the skinnies together.. they need the company. Babies are really social, and putting them in solitude can stress them out.
Get a syringe and make a water solution of 2 Cups warm water, 1 tsp baking soda, 2 TBS corn syrup. Feed them as much as you can with the syringe. They should pick right up...
Sometimes we put 1 dish and/or 1 bottle to a cage and think it's enough because the bottle may seem big, but it's the roller nozzle that is the problem. It just doesn't give out enough, and one rabbit ends up sucking on it the most and the others have to wait too long or stuff their noses underneath, hoping to lick some stray droplets! It's sad!!!
Hope this helps!
Cricket