Problem creating movie clips

I have a piece of code which dynamically creates a movie clip and does stuff with it.

I have 2 movie clips which are EXACTLY the same, except in name and picture.
This code works:
var mb = _root.attachMovie(“pwrupSpeed”, “pwrupSpeed”, 1, {_x:block._x, _y:block._y});

This code doesn’t:

var mb = _root.attachMovie(“pwrupMultiBall”, “pwrupMultiBall”, 1, {_x:block._x, _y:block._y});
And yes, the 2 movie clips are exactly the same, this includes linkage properties…