Code won't display exported MC

Hi,

The following code won't display my exported MC:

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var tractorClip:MovieClip = new scrollTractor();
tractorClip.x = 500;
tractorClip.y = 720;
addChild(tractorClip);

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scrollTractor is the class name given in the library MC properties as linkage.  No, the MC has no instances on the stage.

Any ideas?

Regards,

Ronald