a. invert cradle/candle holds on bar. once you master this, work on holding it with your body not touching the bar ( towards a true front lever, honestly I'd like to see 15 degrees forward of the bar ) can be down on a low bar or pullup bar ( if there is enough clearance ).
The issue is most likely some combination of the cast not being high enough and not dropping the chest back early enough. Unlike a back hip circle, where the shoulders essentially don't move until the hips contact the bar (technically just before the hips contact the bar), in a free hip the shoulders must be pulled back aggressively on the way down from the cast, without waiting for the hips to get back to the bar.
Sometimes I think we do kids a disservice when we teach them back hips as a progression towards free hips, but I can't really think of any alternatives I like better.