Report Writer Question - Breaks when attempting to join a table

We have a report that will show any open replenishment purchase order lines. We are trying to connect this with the shipment table - to only show items that are currently not in stock but are on replenishment purchase orders. 

When I try to join SOShipLine or SOShipLine Split - the report breaks. I believe I am using all the common fields to join them, but not sure why it breaks. 

Could you please advise what I am missing? I have also attached the rpx file. Thank you in advance!

@lauraj46 - Hi Laura, any ideas or recommendations would be helpful! Thank you!