onRelease problem [createmoviclip]

I am having a problem assigning an onRelease handler to thumb_mc which is created using createEmptyMovieClip


	createEmptyMovieClip("thumb_mc", level+1 );
	myMCL.loadClip("thumb/" + stuff[1] + "a.jpg",thumb_mc);
	_root.thumb_mc.onRollOver = function()
	{
		this._alpha = 50;
	}
	thumb_mc.onRelease = function ()
        {
	myMCL.loadClip("pic/" + stuff[2] + ".jpg",main_mc);
}
		
_root.thumb_mc._x = 650;
thumb_mc._y = 50;

the thumb_mc._x command works and the clip shows up on screen, why wont the event handlers work?