When it is inserted it might be coming in as "Fixed". If this is the case you will see (f) in front of the assembly name in the tree. Make if Float then add the apprpriate mates.
If you want the motion of the subassembly to also be available in the top level assembly you will need to click on the subassembly, go to its Properties, and in the lower right corner of that window select "Flexible".
- - -Updraft