I have to save an Inventor assembly to a step file in order to import into AutoCAD as a rendered image. Unfortunately some parts of the assembly go to 0,0,0,in Autocad instead of keeping to their original position in the Inventor model. What do I need to do in the Inventor assembly to prevent this....?

I have to say that the Inventor model is a third party model received as a step file and that I have added Inventor parts to it. It appeared to work ok when I added single parts but then with some subassemblies individual parts fly off to 0,0,0 in AutoCAD.

Thanks