BOM Determination in subcontracting process

Hi MM experts ,

I am new to SAP MM,I have created Bom of semifinish material -A with usage 1

in ME11 the info category for this material is subcontacting .

there is only one bom for this material A,

when I created PO in ME21N tcode with Item category -L the bom is not determine automatically with given Quantity .

Can you please guide me what is wrong .

Regards

Viki.