Why is the Remove Rank not available on a work Item in a plan?

RTC 6.0.3

Developer has an Iteration plan using the Simple Tree plan view. All the work Items in this plan have the Remove Rank Option enabled with the exception of the DRs.  The DR work Item is not available in the project his plan exists, the DR WorkItem resides in a different project however the work Items in the project the plan exists links to DRs so the DRs are in the plan.  What causes the remove rank option to not be available for a work Item in a plan?