The Campaign level reporting only works for Single mailings.

Recurring emails was will always show  "n/a".
 
You will need to drilldown into each occurrence to ret rive the data for the recurring mailings

This data should be available as long as the occurrence is less than 6 months old.
 
If this is something you would like to add as a suggestion for a future enhancement, you can submit this suggestion at our Idea Bank:
http://sphere.uservoice.com/forums/143074-sphere-product-whiteboard