Flash library images not displaying on stage

Hi

I'm working with a Flash CS3 file that I created a while ago. I published the finished movie and that looks as it should.

However, when I go back to the most recent fla before I published the swf, some segments of keyframes and tweens on the timeline don't display on the stage. Even though the keyframes and tweens look fine in the timeline (ie no blank keyframes etc.) I can only see their symbol registration point on the stage. If you need a screenshot or the zip, I'd be happy to send it to you.

The symbols are blank. Is it that I need to re-import the images from the library? I would rather not have to do that.

There seems to be a workflow issue with Flash. I lose track of how many times I've hit undo and then I find that I've undone too much!  I wish there was a clearer or more structured way to go about problems like this.  

Why is this happening and how do I get the appropriate content to appear on the stage?

Import the images, again... right?

Any help is much appreciated.

Thanks!