I used Ninja Ripper to get a mesh for a dress, but when I unwrapped it in blender, it chose to unwrap random faces of the mesh into their own separate islands. I tried putting seams along the edge of the dress to encourage the program not to do that, but it made no difference. What causes this to happen and how do I fix it?
The mesh had disconnected vertices. (For anybody who is interested, it looks like Ninja Ripper simply doesn't connect vertices that go along seams, which is why I was able to get a UV unwrap like the image above with no seams. However, it doesn't find seams perfectly, so you wind up with some faces not attached to anything in the UV editor).
I solved the problem by going to the edit layout, selecting the entire mesh, and using "vertices->merge->merge by distance" to bind all the loose vertices together. Then I manually recreated the seams where I knew they should go.