How to set up MRP control parameters?

HI,

I am a sap new guy.

I wanna know, after planned order was converted to production order,and now BOM changed, we added new materials into production order.

Then we run MRP(material number for order ) by MD02, why new material has no demand ,no PR?

BUT if i run MRP by this new added materials(raw material),it will has demand,why?

I set up MRP control parameters like this 1-3-1-3-2.

Regards

Nemo