Partial shipment for promotional item

Hello

I create a modifier with an item for 30qty get 1 other item free with the setting automatic,i create sales order and choose the modifier item with 60qty ,it automatic create new sales order line promotional item with 2 qty(30qty get 1 free modifier),after booking and release sales order.when i go for shipping the original item 30qty(30 remaining),system ship the original 30 qty and 2 qty promotional item,but the promotional item should be shipped with the proportion of original item(30qty original item and 1 qty for promotion item) but system shipped all the promotional qty irrespective of original item qty