Dialogs are all a little weird and I'm still not sure I've been using them the 'correct way' (the way they're used in the main campaign at least). But they work one way or another in my mod so I'm happy. Whether the NPC is listed as Global or not seems to modify how the StpryEditor handles StartDefaultDialog calls. I don't get it, but I'm kinda sucky at all this. I'm sure others have figured it out.
In any case, in lieu of any better solution showing up, try using this code to ensure the game grabs and starts the dialog (or any dialog) correctly:
IF
DialogStartRequested(CHARACTER_specific_npc, _Player)
THEN
DialogStartTwoSpeakerDialog("name_of_dialog",_Player,CHARACTER_specific_npc);
Throw a dialog into the DefaulDialog section of the sidebar (required so that the speechbubble shows up I think?), but you can theoretically apply any dialog you like to the THEN section of this code, it doesn't have to be the Default Dialog.