Run the cleanup operation on both objects to ensure that there are no issues, also ensure that the 2 bits of geometry are intersecting correctly.
Get the free cartoon dog tutorial from here as it goes into this quite well.
Chris (formerly R@nSiD)
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When the power of love overcomes the love of power the world will truely know peace - Jimmy Hendrix
Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)