Getting some testing/necropsy is best to figure this out if you lose another.
If possible, get a fecal float through your vet this week to see if worms and/or Coccidiosis are an issue.
If not possible, then consider deworming them and treating for Coccidiosis.
Corid and Safeguard can be...
Looks like whole loose feed, do you provide grit as well?
Often birds will only pick out the things they like, for instance all the peas, sunflower seeds, etc., leaving the rest. The vitamins are added as a powder and the birds need to consume all the ingredients to see the full benefit. Some...
Welcome To BYC
I'm sorry that you're having trouble.
Can you please post photos of the birds, their poop and of their housing?
I do agree, sending in a sick bird or one that has just died for diagnostics/necropsy is the best way to find out what is happening so you can make corrections (if...