So in max, I do some general unwrap of an object, and I have a mostly cohesive unwrap, with a few polys that got mapped weirdly.

So I move them into position with the rest of the unwrap. I use snapping to get all the points to align properly. Then I select all of those points (both on the main body, and the detatched polys) and click Tools>Weld Selected.

Sometimes, this actually welds those points together, and I get white lines between them, and there is only one UV point at each vertex to move around.

Other times, it just refuses to do anything.

I honestly can't tell that I'm doing anything differently, or that my mesh is set up differently between the two.

Often, those points that refuse to weld with weld selected, will weld with target weld. . . .

Anyone have any idea why max refuses to weld selected UVs at times?

I don't think it would bother me nearly as much if I could just understand WHY. . .

Thanks

Probably because while you've snapped the U and V values to the same place, the W values may be very far apart. For some reason this affects the weld (it probably shouldn't - maybe I'll look into a script to improve the weld).

You should be able to select all and move their W values to 0 using the type-in field in the bottom left of the Unwrap UVs window. Then Weld Selected should work fine.

Ah! good call!
Works like a charm! thanks Mop!