Histrorical stock data cannot retrieved correctly from MARD/MARDH

Dear MM gurus.

I'd like to retrieve pas stock data from MARD/MARDH table,

but I cannot retrieve number correctly.

For example, if the current period is "May", and

- Feb: GR 100 qty

- Mar/Apr: No goods movement

- May: GR 1 qty, GI 1 qty

(= So, qty in May is 100 qty)

In this case, when I try to retrieve data of Mar, there is no data in MARD/MARDH.

I can retrieve only

- 0 (Jan)

- 100(Apr)

But, in fact, qty in Mar should be 100.

Is there any good way like using Function Module?

BR

Yoshi