I used one assembly to build another assembly. The first assembly included a free parts that could move but in the second assembly they were fixed and couldn't move.
I would like to help/
You have to right click on the sub assembly in the feature tree and select edit assembly. Then you can make any change you want in the sub assembly.
In the second assembly, right click the first assembly and go to "component properties"
Near the bottom right corner of the dialogue, select "flexible" in the "solve as" box.
This will let you move assembly #1 from within assembly #2.