it might be out of range for removal - are you using
getNextHighestDepth() when you originally attach it - and possibly
some components as well>. (I don't actually know what might
happen if one of its child clips are assigned an 'out of range'
depth - whether you can remove the parent clip or not.)
If the trace gives you an extremely high number or a negative
number then just try something like:
mcStack.swapDepths(9999); //a depth where no other movieclip