I dont think its matter of luck, iirc theres some "feature" of the game where chain finish can never deal finishing blow unless its a fixed damage finish, and will always leave the boss at 1 HP. Its like how the mouse cant finish off a boss in kh2 until you revive sora, except here its skill chains cant kill bosses in the same turn. I think I've seen it happen a couple of times in the equipment dungeons.

Maybe you could contact customer support and ask them about it, if I messaged them about everything even if pertinent they wouldnt mark it as high priority if its just one user asking about it lol.