Hey there Colum
Probably it's the Flash Player 10 bug. Captivate doesn't know
how to handle the Flash Player 10 in preview.
Try previewing in the web browser (press F12) and I'd be
willing to bet a dollar against a donut that the full motion will
finally show.
It should also work just fine in the final published version
too. Only the preview is affected to my knowledge.
Cheers... Rick