Also, Please read SAP note 310886 - Line items: Dynamic selections ignored.
This note explains that only fields from certain tables can be added.
FBL1N only fields from: LFA1, LFB1 and BSIK FBL3N only fields from: SKA1, SKB1 and BSIS FBL5N only fields from: KNA1, KNB1 and BSID