If you export from Poser (which I assume you're doing), make sure to check "weld body parts" and nothing else. I'm not sure I understand correctly, but it's sounded like you export from Poser unwelded, and try to make Max do the welding on import. If Poser rounds some vertex positions inconsistently on export (which, knowing Poser, is almost a given), there's a certain chance that Max may miss something.

-- I'm not mad at you, just Westphalian.