When I started getting into intermediate skill level I had a similar problem. Toe edge carves were nice and smooth, but heel edge felt shaky.
What I noticed I was doing when I went into toe edge carves my body naturally tended to lean forward downhill through the carve keeping even weight across the board. When transitioning to heel edge I had a tendency to lean back a bit without thinking about it. This backseating would cause my board to slide out from under me through the carve which is what I think is happening to you?
After focusing on keeping even weight distribution by leaning forward a tad through the transition the problem for me disappeared.