Amazon only lists the rank for the categories and subcategories in which a given item has a relatively
high rank. The category bestseller lists only list the top 100, so if it isn't there, it isn't in the top 100. An item not in the top 100 might still be high relative to the number of products in the category (hence why books, Blu-Ray movies etc. will sometimes show ranking even if it isn't within the top 10k) but if it isn't it won't be listed, and if it isn't in the top 100 it won't be on the category page.
RankTracker.com can provide useful results, but can also be frustrating -- eg. it only does the main categories, as in this case "Electronics" which provides such diffuse data it's hardly worth noting. There may be other tools out there with which I am unfamiliar, but as far as getting a better/broader list directly off of Amazon, I do not believe there is a way.