[V8][POS and MRP: how to correctly create the bom?

Using MRP with POS seems a little bit complicated.

I have a finished product, with a 2 level bom:

Finished product
1 -- Row materials
1 -- Semi finished product
2 ----- Row materials

I'm taking orders using pos and I would like to start automatically procurement procedure and manufacturing process.

I've tried with JIT module to automate the process but my system simply doesn't "listen" to me both with products made with manufacture and stock to order as supply chain information. enable or not mrp scheduler doesn't make any difference.

How do you manage it?

On SAP was quite easy to do...