If you're talking about her spawning into the tv or the mirrors; that's not her spawn.  She starts off with the same spawning behavior as any other ghost where she has a spot in her haunted room that she spawns; but then her hunt behavior kicks in and she teleports to the tv/mirrors.  If she shows up at the mirror or the tv, you're already too late on the swing because she is now in hunt mode and not about to begin hunt mode where the dagger is actually effective.

Edit: Just took a picture for clarification.  She doesn't have the model right now because I'm on nightmare; but this is a Bloody Mariam.  Her spawn room is the basement, so this is where I'd hit her with the dagger to capture.  If she's in the living room, the bathroom, or luna's room; I'm already too late because shes then in hunt mode instead of preparation here.