Transform Data Suddenly No Longer Available

I have a report I created several months ago with its own unique data that spans multiple tables from Dynamics, a static local table and a Sharepoint list.

I recently opened it to make some edits, including how some of the data is transformed only to find that I can no longer access transform data - that option is greyed out.

As I said, this data set was created and shaped when I created this report. It is not based on another existing set. I can't figure out why it happened or how to fix it. Any ideas?