Preloader question - movie already half over!

I have a preloader which loads an external swf (an animation I created in flash)
This works fine, except that when the movie plays, it’s already halfway through.
Here’s the code…

var myContent:URLRequest = new URLRequest(“MyAnimation.swf”);
var myLoader:Loader = new Loader();

…add the listeners, functions etc - all working ok

function loadComplete(event:Event):void
{
addChild(myLoader);
}

I think I must be missing something fairly basic…I’ve searched but can’t find the answer. I just want my animation to play from the start, once loaded. The swf is about 4 meg.