Revit :: Why Dropdown Boxes Might Get Locked Out In Schedule

I have certain parameters I'm using in a Door Schedule that should offer a drop-down list of options.  When I click in the schedule cell to make a change, I don't get the drop-down.  If I select the same door element in the model (such as in a Floor Plan view), I get the drop-down in the Properties window.
 
One parameter is a Shared Parameter, which is Instance-Based called "Panel Type", and is the "<Family Type...>" kind of parameter.  The other troublesome parameter is a Project Parameter from a Schedule Key (Key Name) for a predefined list of Hardware options.  Again, the drop-down box doesn't appear in the schedule cell but does appear in a model view on the Properties window.
 
Strange enough, if I create a new Door Schedule from scratch and add these same two parameters, the schedule behaves properly and offers the drop-down box.
 
why these drop-down boxes might get "locked out" in the schedule?  I don't mind re-creating my Schedule once, but I'd prefer to understand what and why Revit is behaving this way to avoid this in the future.