Dear David 

did you got solution to your problem 

I also got stuck with sub assembly product stock on hand is not considering the MO for c item

I've tried with two different configurations.
Supposing that I have a Product C which is comprised by Product A plus Product B.

In other words. A+B->C

The BOM type is: Manufacture this product.
BoM: 1 Unit of A + 1 Unit of B = 1 Unit of C

The idea is that the process be as follows:
1. I have 10 units of Product A, 10 units of Product B and 10 units of Product C already in stock
2. I make a Sales Order for 30 Units of Product C
3. The system will check the stock of product C, therefore the system will reserve the 10 unit I already have in stock, and the other 20 units will be executed by a Manufacture Order (MO).
4. The manufacture (MO) order will check the availability of Products A and B, reserving the 10 units of Product A and Product B that I already have in stock, and then will make a RFQ to my vendor for the remaining 10 units of product A and 10 units of Product B. It would be even better if I could execute the MO for those 10 units initially reserved while I wait for my vendor to bring me the remaining products.
5. One the purchased of the remaining units of Product A and Product B is completed, those will be processed by the Manufacture order to create the last 10 units of product C.
6. Process the delivery to my client. It will be even better if I can choose delivering my product C as they are being manufactured (for example 3 batch of 10 units of Product C).

These are the two configurations that I tried.

Configuration Nº1: Using the route Replenish on order (MTO)
Item A: Storable; Can be Purchased (with vendor setted); Routes: Buy + Replenish on Order (MTO)
Item B: Storable; Can be Purchased (with vendor setted); Routes: Buy + Replenish on Order (MTO)
Item C: Storable ; Can be Sold; Routes: Manufacture (BoM setted) + Replenish on Order (MTO)

Then I understood that Replenish on order (MTO) route will buy (or manufacture) the quantities ordered in Sales Order regardless of each product's stock. Therefore, this option is not suitable to my case.

Configuration Nº2: Using reordering rules
Item A: Storable; Can be Purchased (with vendor setted); Routes: Buy
Item B: Storable; Can be Purchased (with vendor setted); Routes: Buy
Item C: Storable ; Can be Sold; Routes: Manufacture (BoM setted)

Reordering Rules:
Product A: Minimum 5 (I even tried with 0) and Maximum 10
Product B: Minimum 5 (I even tried with 0) and Maximum 10
Product C: Minimum 5 (I even tried with 0) and Maximum 10

This configuration seems more logic to me. But when I execute the sales order, the system just reserve the 10 units of Product C that I have on stock and it do no more actions (neither the Manufacture order nor the Requests for Quotations) no matter how many times I run the scheduler.

I believe that the problem is that the reordering rule of product C don't trigger the Manufacture Order, because I even tried to run the scheduler with a reordering rule Min 15 and Max 20 por product C, having 10 units on stock, and when I ran the scheduler nothing happened again. And I can confirm it's not a BoM problem because if I create a Manufacturing Order of product C manually, the rest of the process (Request for Quotations and Manufacturing) work very well.

Thanks in advance for your help!