Skin Clusters break sometimes. Or you just want to copy skinning from one character to another.
Usually when I have a skin cluster playing up I rebind a duplicate of the mesh to the joints and copy the skin weights from the original mesh.
To do this:
1. Duplicate mesh
2. bind duplicated mesh to the same joints as the original
3. copy skin weights over from old mesh with Skin > Edit Smooth Skin > Copy Skin Weights
Gives you a fresh skin cluster but with the skinning you’d setup already.