Production Plan is ignoring already assembled sub-assemblies in stock

Oke, so… I don’t know how to name this topic correctly,
I’m quite new to ErpNext (and these kinds of systems in general) and I’ve spent the past 3 weeks or so customizing the system, adding stock and BOM’s, etc.

but here’s the thing:

How do we manage the manufacturing of a multiple of different items, all with multi level BOM’s, where it can be the case that some sub-assemblies are already in assembled state in stock.

So for example: Item A is made of B and C, where C is made of E and D. I want to make item A twice and I have 1 item C assembled already in stock. (and 2xB, 1xD and 1xE). So I have everything to make item A twice.

But right now when making a Production Plan, getting the items from the sales order and then clicking ‘Get Raw Materials For Production’ it is telling me that I need to order 1xD and 1xE. It is completely ignoring that 1xC that I already have in stock.

Why is this happening?

(Some context: we are building camper vans. So you can imagine all the furniture and accessories is made of a lot of different sub-assemblies. All the parts needed come from a lot of different suppliers. We don’t want to wait for everything to be in stock before starting the build of an entire van. So while waiting for materials we spend out time assembling sub-assemblies as much as we can with what we have in stock.)