All of a sudden my animation wont work, what am I doing wrong?

I have been working on an animation. In it letters transform into simple squares. I did 4 of them without a problem. On the last two, using the same techniques I have used for the four other letters I cannot get the animation to work. What happens is the letters move around properly but the squares do not join in so the animation doesn't work. I have done all I can think of including replacing the letters to start the animation and replacing the squares to remove any 'contamination' from unremembered errors on the work. Why did the work techniques work four times and then stop working. I have closed down Flash completely and started it up again. No go. Help!