Precedes Relationship and Gantt Chart Problem

I am creating a work item set with various Milestones, Events, CRs, Tasks and Meetings which are all children, grandchildren (and so forth) to the Build Release Milestone. There are many sibling, and even some cousin work items which have the predecessor/succesor relationship. When creating a plan, my Gantt chart shows some of the scheduling dependencies (from the predecessor/succesor relationships) correctly in that a work item scheduled time does not start until after its predecessor is complete. However, many of my work items which do have predecessors are starting right at the beginning of the iteration rather than being scheduled to start after the end of their predecessor's estimated completion time! Any ideas?

Thanks!