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.
I'm not sure if this is your issue, but when creating in context parts, if you save the assembly under a different name it will make the reference out of context. Once it is out of context, the in context relations won't update with the assembly. Make sure you save out the assembly under a name you will keep, before creating then incontext parts.
Alternatively, if the assembly is not referencing that document, then it will not update. If you're sure the assembly is referencing the correct document, then try hitting the rebuild button when you're back in the assembly and seeing if it updates. Are you able to attach the files so that we can take a look?