Makes more sense now...you probably have soem groups that have no records wherer the transaction_type="Withdrawal" hence nothing to add at all. A little tricky to tweak without seeing the actual data but you can convert this from a RT to a SUM and that should do the trick.
Change your formula field (used in the RT) to
if isnull({table.amount}) then 0.00 else if transaction_type="Withdrawal" then {table.amount} else 0.00
Place it on the detail row and validate that you get 0.00 everywhere other than the rows you want to include.
Just SUM this field instead of using it in a RT. Assuming the group level has at least one record it will have at least one field to SUM on and therefore should include the 0.00 in each group.
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