Added a new bone to armature but it won't deform mesh

I’m almost certainly making a very stupid mistake here. On some of my animations, the character’s leg is clipping through the shirt hem. To fix this, I’m adding a small bone to my armature (hemback.L) to pull one of the vertices of the shirt away from the leg. I parent the bone to the hips and weight the vertex to it 100% (with deform checked). But then when I animate the bone, it has no effect on the vertex. Any insight into my ignorance is greatly appreciated.

The selected vertex has only a tiny amount of weight on the pelvis vertex group, and your mesh contains no vertex group at all for hemback.L. You don’t parent a vertex to a bone; you give it weight in a vertex group with the same name as the bone. Right now there isn’t one.

Thanks for taking a look at it. I’m not quite sure what you’re saying though – I was under the impression that vertex groups are more or less a bookkeeping feature and did not determine interactions unless you specifically say that you want a group to have a certain weight. I have not been creating vertex groups at all, I’ve just been weighting the vertices individually, and everything has been working up to this point.

EDIT: Ok, I got it working, and it seems like something screwy may have been going on. Every time I tried to weight that one vertex to the new bone, pop into pose mode and then back into weight paint, the weight would be on the hips instead. After about ten Blender restarts it seems to have stopped?

No, vertex groups are what creates the influence for the bone. That’s why they have the same name as the bone. Vertex groups aren’t bookkeeping, they’re the glue that holds the armature and mesh together. Weight painting is just a visual representation of the vertex groups’ influence. When you parent an armature with automatic weights, Blender creates the vertex groups for you. That’s why you never created any yourself. When you weight paint, you’re just using a brush tool to change the values of the influence of vertex groups upon their member vertices.