Hey Glenn,
The quick version is that extra credit does not work in a category or sub-category if that category or osub-category is set to be Weighted means. If the category or sub-category you want to add extra credit to is Sum of grades or Simple weighted, then extra credit will work. So if your top-level category is Weighted but your sub-categories are Sum or Simple weighted, then you can only add extra credit to those sub-categories.
The long explanation of why this is that when you use Weighted, you are dealing with weights, not points. Although they both appear as numbers, there is a difference. Weights are weighing points vs other points. So you could have a 1000 point category that is worth 5% of the overall grade and then a 10 point category that is worth 95% of the grade. Using the previous example, when comparing points from one category vs points from the other category, they are not equal. If those categories are set to Sum or Simple weighted, when comparing one point in that category vs another point within the same category, those points are equal.
Looking back at my explanation, I may add more confusion to the topic but that is how it functions.
Happy
oodling!