Вы, вероятно, пропускаете Nz :
SELECT
drvYourTable.TransDate,
drvYourTable.DrAmount,
drvYourTable.CrAmount,
(Select Sum(Nz(DrAmount, 0) - Nz(CrAmount, 0))
From tblYourtable
Where tblYourtable.TransDate >= drvYourtable.TransDate
) AS Balance
FROM
tblYourTable AS drvYourTable
ORDER BY
drvYourTable.TransDate;