4 week average calculation having missing values on bar chart because of lack of events in 1 week

This may be a confusing issue seeing it's quite specific so apologies in advance.

I have a calculation that tracks the 4 week average of an event's occurance, grouped by weeks. This event only occurs 0 to 3 times a week at a given place.

The calculation works fine on weeks that have had events (e.g. Week 6 in later posted screenshot), even calculating correctly when the 4-week calculation includes the 0 event week (Week 5), but returns blank when based on a week that did not have an event but has a non-zero 4 week average (e.g. Week 5).

Here's my calculation:

4WkAvSiteGraph = 
VAR _LastProdWeek = MAX('Date'[ProdWeekOrder])-1
VAR _Duration = 4
VAR _CalculationPeriod =
    FILTER(
        ALL('Date'),
        AND('Date'[ProdWeekOrder] > _LastProdWeek - _Duration,
            'Date'[ProdWeekOrder] <= _LastProdWeek)
    )

VAR _EventsInSpan =
    IF(
        COUNTROWS(_CalculationPeriod) >= _Duration,
        CALCULATE(SUM('DataTable'[EventCounter]), _CalculationPeriod)
    )

RETURN _EventsInSpan/_Duration

Here's the bar chart visual of the 4 week moving average, note the missing y-axis values like Week 5, with a line chart underneath it showing the basic count of events that have occured in the same weeks at the same location:

Why does the calculation return a blank for these eventless weeks that have a non-zero moving average, and how do I modify the calculation to stop this from occuring?