I am experimenting with in context modelling.
I found that in order to get part drawings for manufacture, I had to first create real/external documents of the parts.
When I subsequently wanted to amend a part, I edited the external document, expecting the amendment to show in the top assembly.
It did not.
By opening documents in a particular sequence, I persuaded the amendment to show in the assembly and I saved the assembly, but the amendment did not show on next opening.
I guess I am unaware of an important rule.