Does the clothing object consist of multiple elements in Max? In other words, did you use Garment Maker in Max and then collapse to a mesh/poly object? Especially the look of the armpit probem makes me think of that.

I'd love to see a wireframe of the problem object, then I can see whether the vertices in the sleeve have matching vertices in the body of the shirt. Poser can only weld vertices that are in exactly the same place, there's very little tolerance and the tolerance is not adjustable.
I find it useful to build a clothing item out of just one single mesh element, and then cutting it up in Max. The "one element" approach makes sure that Poser will be able to weld the parts.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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