UDF Does Not 'reset'

My UDF

FUNCTION [dbo].[fn_concat_boxes](@item varchar(10), @week int, @type char(1))
RETURNS VARCHAR(100)
AS
BEGIN

DECLARE @Output varchar(100)

SELECT @Output = COALESCE(@Output + '/', '') +
CAST(quantity AS varchar(5))
FROM flexing_stock_transactions
WHERE item = @item AND week = @week AND firm_or_commission = @type
GROUP BY quantity
ORDER BY quantity

RETURN @Output

END

gives the same output every time it is called, despite different parameters passed. @Output does not change as expected.

Any ideas guys?