Are you still within 30 days of the item's last estimated date for delivery, then you need to be careful to open the correct case.
If this was one order and you only received half the order, you need to open a case for item not as described, and select the reason.... Missing Parts or Pieces.
If these were 2 separate orders, then open a case for item not received.
To open a case, go right of the item in your Purchase History and from More Actions open the relevant case.
@amachia44