WoL Any way to make Spirits good candidates for level up training?

[Zarxrax]

What causes some spirits to get a big benefit from Doc Lewis' training? Is there a way to make spirits into good candidates for the training?
Obviously, I would like to use him as much as possible for free level ups instead of using my food.

 

[JiggyNinja]

I don't know if it has a criteria, it might just change randomly. I had Dixie Kong once with the blue chevrons, and after a while they went away. I did send her on an exploration, but I dn't know if that had anything to do with it.