Hi
Please check the following things :
1. In Material master MRP4 view - Field "Individual/Coll" should have value "2" or "Blank" & In BOM (Transaction code CS03) item detail section - Field "Explosion Type" should have value "LB" ("Individual/Coll" should have value "2" or "Blank") .
2. PIR should have PIR Info category "3" , i.e., Subcontracting PIR must available for header material which we receive back from subcontractor.
3. Special Procurement Key must be "30" in material master MRP2 View.
4. Purchasing Item Category must be "L" and production BOM must be maintained, if you want to explode BOM in PO.
5. Please check BOM valid period is effective from in Tcode CS03
6. Please check your PO date against the valid date of the BOM valid period.
Regards
Rahul