The two tanks shown here were built from the same treads. The left hand one, Tank 'A' was built first. I duplicated the document, deleted the tank body, cloned the treads to make double width, and added a new body. Voila! Tank 'B'.

Both run fine in their respective docs. Both have been dragged to the browser. Tank 'A', when inserted into a scene runs fine. But note the sequencer for Tank 'B'. The left hand time-track is from it's home doc. where it was built. The right hand one is from the scene in which I want to use it. Where did all the keyframes go?