There are a couple of different possibilities:
1. In order for holdings to display accurately in WorldCat.org, a library must have both a FirstSearch/WorldCat Discovery subscription and a Cataloging subscription with OCLC. Without both of these subscriptions, holdings will not be visible in WorldCat.org.
2. Due to the format/edition grouping feature, an item's detail page will not necessarily show the edition on which your library has a holding. To show holdings for all editions of the same resource, toggle the link at the top right of the list that will read either "just this edition" or "any edition."