Scrap not generated in

Dear experts,

Fg bom has scrap code with negetive qty in ck11n.

What i did it

1) I created seperate valuation class for scrap sale with standard price

2) Assign scrap GL for scrap valuation class in obyc with GBB-VBR and GBB-ZOF

3) Same Gl is assigned to VBR and ZOF

4) Then did the standard price release

5) created cost component SCRAP in order to show scrap seperatly in ck11n or Production order

But at the time of MB31 or GR no scrap is generated.Only FG material is produced.

My questions are

If i am running ck11n for scrap code.System told me to assign Gl to GBB_VBR

but when i am running the ck11n for FG. where scrap as defined with -qty in FG-bom.System guide me to assign GL to GBB-ZOF

then i have assigned same GL to ZOF also.

Why i have to assign scrap GL to ZOF in obyc?.

Why scrap is not generated in GR.only FG is produced?

M i missing Some thing?