Otherwise if that's in place you can try going back to the headjoint if they're younger players. There's a tool called a Pneumo Pro that can help with kids understanding where to visualize the air.
To add to the harmonics it sounds like you're already exploring, Kathi Blocki released a really cool packet of Jingle Bells with changing fingerings with the harmonics to keep getting Jingle Bells out. (I was totally geeking out over it when I bought it) I would consider that an 8th-12th grade tool. (Younger if they study privately, which I'm guessing these don't)
If none of those solutions work see about getting a clinician in, or zooming with someone. There are a lot of other things it could be. This year with COVID it's not making it easy to explore all of the problems we're seeing with embouchures!