Can you explain what you did to create the symbol as an animation? What type of symbol did you create - it should be a MovieClip (usually)?
OK - steps were:
1. Create text object on main timeline.
2. Add some frames and create motion tween
3. Create some keyframes by moving object around at different frames
4. Select object and hit F8 to convert to Movie Clip.
Hmmm. That seems to work now with a test movie, not my original file, which I rebuilt in the end. If I can reproduce what i think happened before I'll post here again.
Thanks for your help
What you might try doing is to create the (empty) movieclip symbol first, then build your animation inside it.
When building timeline motion tweens one key thing to remember is that the same object at the start of the tween must be the same object at the end of it.... you can change its position, color, transparency, etc... but it has to be the same object that started the tween.
Yes I think in future I'll automatically create animated objects inside Movie Clips.
In this case, it was just that I'd already spent a fair bit of time tweaking an animation to pixel perfection before deciding to use a movie clip. Ah well.