Oof, that's rough.
Working with unpaid artists, especially minors or other inexperienced people, is always a bit of a minefield. People often commit to things when they'e not really good at gauging how much work it'll be or how long it'll take, and then they get stressed out, realising there's no way they can keep up with the responsibility they've taken on, but they're too scared to say "sorry, I misjudged this, I'm going to have to back out" they just ghost instead.
You might not really have done anything wrong, you were just working with artists who weren't ready for the level of responsibility.
Try to look for artists with a fairly large portfolio of work over a number of years, ideally adults, and talk through what you want to make. It might be best to start small like "let's try making a pilot episode first to see how well we work together". If the artist finishes the episode, but doesn't want to work further, you'd at least get a portfolio piece out of that, and so would the artist.