Why can't I see some items in a collection?

Collection members see only the Conversations, profile and group walls, Social Objects, and other collections they can access. For example, Jane has access to the Conversations Incident Reports, Incident Statistics, and Incident Cost Analysis. Jane's coworker John has access only to the Incident Reports Conversation. All of the incidents Conversations are part of the collection Incidents. Jane and John are both members of this collection. In the Incidents collection, John can see only the Conversation Incident Reports, while Jane can see Incident Reports, Incident Statistics, and Incident Cost Analysis.