DAX formula not returning a value for every day

[rudivs]

I am calculating a fund balance (essentially account balance) as a measure:

Fund Opening Balance =
    CALCULATE(
        SUM('Fund Balances'[Opening Balance]),
            FILTER('Fund Balances','Fund Balances'[Year] = YEAR([First Day of Previous Month])),
            FILTER('Fund Balances','Fund Balances'[Month] = MONTH([First Day of Previous Month]))
    ) +
    CALCULATE(SUM('Fund Balances'[Movement]),
            FILTER('Fund Balances','Fund Balances'[Year] = YEAR([First Day of Previous Month])),
            FILTER('Fund Balances','Fund Balances'[Month] = MONTH([First Day of Previous Month]))
    )

where 

First Day of Previous Month = STARTOFMONTH(PREVIOUSMONTH(Dates[Date]))

Basically, the opening balance of this month is equal to the opening balance of the previous month + the movement in that month.

This calculation works, but does not return a value for every date, as can be seen in this table:

This results in an incorrect fund balance for many days, since fund balance = Fund Opening Balance + Fund Movement. Any idea what's wrong?

[rudivs · Accepted solution]

I figured out what was causing the problem. One of the relationships with the Funds lookup table was set to bidirectional. Setting it to 'Single' sorted out the missing values.