Problems with rigging- Gorundhog tutorial part4

i got stuck in a part of the tutorial where david noticed that the flur doesnt move with his model so he just placed the armature above all the other modifiers. When i tried to move the shoulder of my groundhug i noticed that some kind of vertex on the forearm just doesnt move and starts to deform my model, i can’t figure it out whats happenins im just new on blender and dont know why that vertex just got stuck there.