When first talking to her she's very hesitant to talk to the player, but after a speech check (50), she'll be more open to the player character. A low intelligence option is also available to get her to talk (3 or less intelligence).
Melody seems to get trapped behind walls easily, making her impossible to reach. On the PC, this can be corrected by opening the console, typing Vorlage:Console to target her, then Vorlage:Console to place her at your location. Doing this in the Brahmin pen will make her walk back to her proper spot.
Melody may not appear at all after reaching a certain point in the storyline.