Strange Problem: Animation Tween

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i am facing small strage problems
i know how to use animation tween
i ahve two layers
the first layer has flvplayback which loads flv
the above layer has square: in frame 1 alpha = 100m in frame 20 alpha = 0
so the flv apears gradually

the animation does not work :confused:
i copied the same frames to another fla file, it works :o

what is happening

i relaunched the flash, but still there
i restarted my PC, but still there

Thanks in advance