Matrix is missing a column

Hi All,

I've done a search and didn't find an answer for what is happening to my matrix.  I have rows of data for each project, and 12 fields in the "Tracker Milestones" Columns data - it is only showing 11 of them.  The one that's missing is "Start on Site".

The matrix comes out like this (missing the "Start on Site" column):

There are no filters on the visual, page, or all pages:

If I create a slicer, "Start on Site" is not there either...

I must be doing something wrong somewhere along the line, but the data is there in the table? I've tried everything I can think of, so please could one of you lovely, helpful people let me know how to fix this please?

Thanks,

Martin