_global assignment

Im trying to use global variables right now for fields of different things in my game. I declare one global variable on the first frame of my movie and then assign that name to a text box for the user’s name. Then on the next frame I have a movieclip with a dynamic text box in it which is of the same variable name, yet that text box won’t display the user name. I did a trace within this movie clip and the variable im using returns nothing. Why might this be? Are there any common mistakes when it comes to global variables.