Comboboxes from hell

My current project requires me to place a movieclip on the stage using the attachMovie method. The Moviclip contains a couple of comboboxes. A very strange thing occured when the movieclip was attached, the comboboxes dropped down underneath everything else found in the movieclip even though they wer on a layer above!

After one of my usual sessions of trial and error I noticed that if you use a number instead of getNextHighestDepth() within the attachMovieClip method everything works as it should. Do a google search on this problem shows other flash bods have had the same problem.