My betta wouldn't eat for a day or two after I got him, and even then he was very picky on what he would eat and only ate a little bit. He was actually in impeccable shape when I got him. The cup he was in was nearly pristine.
So I wouldn't be surprised if it was because of stress. What I did with Raphael is I fasted him for a day after he was placed into his new tank, and then just kept offering food everyday. Just keep trying, he'll get hungry enough to try something soon. I've known betta's to be able to go a week without food without too much damage, so a few days without food shouldn't hurt.
It's always nice to see the lengths some people will go on here to help animals. It's really amazing that you bought 8 bettas to try and give them a better life.
happy owner of a wild type GFP axolotl named Percival and too many guppies to count.