Blood does not need to be seen for it to be coccidiosis.Could it of been Coccidiosis? I checked the bedding and there was some diarrhea. No blood. I've been giving them medicated feed and the occasional nutri-drench
It will not hurt to go ahead and start treating them for it today.
You should not give vitamins while treating for coccidiosis.
Have you ever had to treat for it or lost birds in the past from coccidiosis?
If so then it is highly possible these chicks are dying from coccidiosis.
Watch this video.
https://www.backyardchickens.com/threads/coccidiosis-video-worth-watching.1262022/