How to tell if the arc is flat
Look at the protagonist's decisions across the outline. If the choices in later chapters come from the same emotional stance as the early ones, the arc may not be developing enough.
Look at what the story is asking the protagonist to risk. If the costs stay external while the inner stakes remain protected, the character can survive the plot without being transformed by it.
Look at the ending. If the climax solves the external problem but does not reveal an earned shift in belief, behavior, vulnerability, or agency, the arc may have stayed too static.