well i noticed in flash that when you duplicate movies a lot, certain MCs get lost or replaced by others in the same depths. Try mc_clip.swapDepths(1000) to see what you get. The depth value of 1 is a little small for my comfort, and remember flash puts higher number depths closer to your eye.
Have it increment the depths. In the same Flash file I have needed help with, I am instancing a movie a few hundred times. So everytime you duplicate it, do something like mcDepth++ or whatever your depth variable name is.