Blender, Mesh Tearing

I’m making an animation in Blender with a mesh that hasn’t typically caused any trouble before. Now, for this certain animation my character needs to bend down and forwards, but when I move the bones, a few vertices of skin stay behind. I’ve checked, and all vertices in that area are part of vertex groups, so I don’t see why they don’t come along. Any ideas?

Maybe you have those vertices asigned to another bone(s) too?

OR not assigned at all.