Sometimes it is not easy to diagnose why they have diarrhea. Many things can contribute to it from a slight imbalance in the intestinal tract, bacterial, viral or fungal infections, stress, chilling, over heating, the food or water, their treats, etc...the list is long. I have a couple of birds that will take loose sometimes. Their systems do put out loose stools sometimes during the day only. I have never been able to figure out how to stop this. I have found that the best time to judge poop is the poop you find under the roost bar from over night. The birds have digested mostly chicken feed, they have been sleeping and not stressed about things, they are quiet and not doing much. This poop seems to be their average output. During the day they may be eating treats, running from predators or what ever it is that keeps them hyper all day. If they are leaving solid stools over night, but still leave some loose ones during the day, I don't get too concerned. But when I find lots of liquid stuff off the roost bar, this is when I need to do something or at least try and figure out what is going on.
Chicks this age can be afraid to try new foods. Once they age a bit, the eat everything first, then ask questions later. LOL Do they like hard boiled eggs? You can try putting some yogurt on some eggs or something else you know she or they will eat.
Keep up with the ACV and it never hurts to get some probiotics to keep on hand. My birds get probiotics a couple of times a week in their water. Helps with digestion and helps to ward of pathogens.