Problem during doing Subcontracting ...

Hi experts ,

I'm facing a problem during doing subcontracting operation ...

for BOM , maintain finish good as main item and semi- finished good as sub item ....

for finished good ( main item ) - main , NORM as item cat. grp , and for semi-finished good ( sub-item) - maintain also NORM as item cat. grp .

For both material ,

special procurement - subcontracting , procurement- external , mrp group - 070 , maintained ...

In BOM creation , BOM usage - production (1) maintained ...

.............. after doing all these settings , the BOM didn't explode in PO when I enter the main material .......

please advice me , how should I proceed ??