Get the Max Available Month on Selection

Hi vin26 -Create a Measure to Get the Max Available Month for Each Employee

Max Available Month =
CALCULATE(
MAX(Table1[Month]),
FILTER(
ALL(Table1),
Table1[EMP ID] = MAX(Table1[EMP ID]) && Table1[Month] <= MAX('DimDate'[Month])
)
)

Measure for last 6 months calc.

Average Last 6 Months =
CALCULATE(
AVERAGE(Table1[Amount]),
DATESINPERIOD(
'DimDate'[Month],
[Max Available Month],
-6,
MONTH
),
Table1[Amount] <> BLANK()
)

check with above measures, if still issue exist please share the pbix file by removing the sensitive data.

Hi vin26 

Please try this:
Maybe you can delete the relationship between the Table1 and the DimDate:

Then add a measure:

MEASURE =
VAR _slicer =
    MONTH ( MAX ( 'DimDate'[Month] ) )
RETURN
    CALCULATE (
        AVERAGE ( 'Table'[Amount] ),
        FILTER (
            ALLSELECTED ( 'Table' ),
            MONTH ( 'Table'[Month] ) < _slicer
                && MONTH ( 'Table'[Month] ) >= _slicer - 6
        )
    )

Then add a slicer with DimDate[Month]:

The result is as follow:

Best Regards

Zhengdong Xu
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