I'd bump the 700 gram size up to 28/34 quart tubs and give them hides till they ate better, do you use hides now?
I only use them with slightly pickier snakes which it sounds like your qualify as...

You say the yb is 3 weeks old
You waited a week to feed it and no go so you waited 2 more weeks and no go.
Did you purchase an unfed brand new hatchling yb?