Computed Weight on product level in BOM

Hello Community,

I have a question regarding the computed weight property on a BOM in a drawing.

When I have computed the weight through B.I Essentials on all 3D-parts in a Physical Product. The Product appear green, and the weight is 100% computed. In the BOM on the drawing (Summarized), all the 3D-parts have a defined computed weight. When changing the same BOM to Top Level instead of Summarized, the Product weight is 0 kg. 

Can anyone explain what I'm missing?

Best regards,

William