I created a calculated field - by summing up values of other fields but after i saved and exited, I can no longer select/deselect fields for the calculation

I was successful in creating some calculated fields using Adobe Acrobat XI Pro, which added up the sum of the values from other fields and saved and exited and sent a copy to my personal computer to edit using PDF reader.  When i re-opened the original file using Acrobat XI Pro, to edit the calculations, the screen no longer displays any fields for selection or deselection for the calculation.  Its blank.  Has anyone run into this problem before? Has anyone figured out how to get around this?