Curate, connect, and discover
This is part of the reason why I don’t believe our ghosts are going to cross over any time soon. While they have had discussions about their deaths and their pasts, there needs to be some sort of healing and acceptance of it, and that hasn’t happened yet for our core ghosts.
I feel like Dawn's scar healed. Like the physical scar Maddie was in did not feel like the other scars. So I think her crossing over kind of healed her scar. And so what I also think is that to cross over a ghost is gonna have to get closure on something that is immediately connected to their death/scar. Because all the ghost's scars are about their death but it's heightened and with all their insecurities added into the situation. If I remember correctly Dawn crossed over because Maddie had said something to her that was the complete opposite of Dawn's insecurities. Dawn's death is directly connected to her friends making fun of her behind her back and her obviously feeling hurt by and insecure about that. Then later Maddie says to her that she deserved an apology and that she's sorry people made her feel like a joke because she isn't and she even tells her that she's a good friend. Dawn was probably always insecure about that. I mean the other ghosts did make some not so nice comments about her too. But when Maddie came she told her the exact opposite and she truely meant it. She gave Dawn the validation that she had probably always wanted from her friends and that healed something for Dawn which lead to her crossing over. That's why Rhonda didn't cross over even after finding out what her friend's song was actually about. Because it doesn't directly connect to her death. I'm assuming for her to cross over she would have to get closure on Mr. Manfredo's betrayal, however that may look like for her.
So what I'm saying is. I think crossing over is directly connected to a ghosts's death and the insecurities that lead to/were present during their death and that are represented in their scars. When a ghost crosses over their scar mentally and physically "heals" because that ghost is not bound to those insecurities or their death anymore