Simple question: flash is stretching my movie files

I can’t seem to figure out the rules of Flash regarding movies in movies, graphics in movies, ect.

Often times, my movies within movies are getting stretched and proportionately distorted when they are dragged to the stage.

How do I avoid this? I make things in Photoshop at 72 dpi and at the size its going to be in Flash, and import to library. And often when I add that image to a movie, and then bring the movie to the main timeline, its too big and has been stretched.

I really appreciate any advice you can offer!