Actually, the mid line looks a bit thicker than the rest, indicating that it's not welded properly, perhaps from the modeling program. Try importing the obj into Poser with "Weld Vertices" checked... Thing is, I can't remember if that'll kill your grouping. You could also select the offending vertices with Poser's Grouping Tool and use the Weld there.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.