Date table not providing all the data

Hi abihoughton27 

I see you have two years data in tables. If you drag only Month column into the visual, data in the same month of different years will be aggregated together and makes the running total wrong.

To fix it, you could add a Year-Month column into Date table and use it in the matrix visual. Or add Year into the matrix as Rows field too. Or if you only want to get running total in a selected year, you can add a Year filter or a Year slicer to select a specific year. Then try the following measure:

RT =
CALCULATE (
    [Baseline FTE Sum],
    FILTER (
        ALL ( 'Date Table'[Date] ),
        'Date Table'[Date] <= MAX ( 'Date Table'[Date] )
    ),
    USERELATIONSHIP ( 'FTE Milestone Benefits'[Current realisation], 'Date Table'[Date] )
)

This article should be helpful: Running Total in Power BI - Goodly. It calculates running total across all months. 

Best Regards,
Community Support Team _ Jing
If this post helps, please Accept it as Solution to help other members find it.