Hello, we also hatch and raise quail. One of the biggest things that I have picked up on is how important humidity is for higher hatch rates. We like to keep our humidity around 50% until a few days before hatching and then increase it.
We give apple cider vinegar to our chicks to help with respiratory and digestive issues. I don’t know if it will help a whole lot since it’s more of a preventative but it might be worth a try? It does sound like it might be a vitamin deficiency.