12 month rolling average - quick measure

Hello, I am trying to calculate the average staffing over a 12 month period so have used the new quick measure to generate this for me, however when I pull the calculation into the report it comes back blank, not sure what I am doing wrong here if anyone is able to suggest anything to look at. Thanks!

[Emp On Date] is a measure which generates me the number of employees active on that date based on their start and leave date

'Start_Dates'[Full Date] is a table of dates linking employees start/leave dates

Emp On Date rolling average = 
IF(
	ISFILTERED('Start_Dates'[Full Date]),
	ERROR("Time intelligence quick measures can only be grouped or filtered by the Power BI-provided date hierarchy or primary date column."),
	VAR __LAST_DATE = ENDOFMONTH('Start_Dates'[Full Date].[Date])
	VAR __DATE_PERIOD =
		DATESBETWEEN(
			'Start_Dates'[Full Date].[Date],
			STARTOFMONTH(DATEADD(__LAST_DATE, -12, MONTH)),
			__LAST_DATE
		)
	RETURN
		SUMX(
			CALCULATETABLE(
				SUMMARIZE(
					VALUES('Start_Dates'),
					'Start_Dates'[Full Date].[Year],
					'Start_Dates'[Full Date].[QuarterNo],
					'Start_Dates'[Full Date].[Quarter],
					'Start_Dates'[Full Date].[MonthNo],
					'Start_Dates'[Full Date].[Month]
				),
				__DATE_PERIOD
			),
			CALCULATE([Emp On Date], ALL('Start_Dates'[Full Date].[Day]))
		)
)