[Tweener] not exactly the tweener problem

[Makc]

I have a movie clip with stuff in fla exported to swc, and then this
swc is used as library in flashdevelop where I make instances of this
clip and use tweener to tween alpha of this movie clip.

the code looks like this:

trace ("this is always shown with correct non-null mc: " + mc);
Tweener.addTween (mc, { alpha: 1, time: 0.5*speed,
onComplete:function():void{trace ("this is never shown");} });

I get this thing working in like 5% of test runs. What could possibly
be the reason?