User Defined Sorting in Plan Items

I am using CLM 6.0.6, latest iFix.

I've found documentation and even other forum questions that reference a user defined sorting option in plan views. All of the references mention that defining the sort is a simple drag and drop of work items to reorder them, and this sort is persisted across views in the plan. For some reason, dragging and dropping work items in a view that has a Sorting > User Defined option added does not change the order of work items. Is there something that could prevent this functionality from working, or am I missing some configuration? Any help is appreciated.