Model's end not following bone

Question

I'm just so new to blender I can't even figure out why this is happening. (picture)

Answer 1

Looks like a weighting issue. This should fix it:

• Tab into Edit mode.
• Select one set of the problem vertices (making the armature modifier work in edit mode will be useful here).
• Go to the mesh properties tab of the Properties panel (the triangle), and find the vertex group matching their bone. Hit Assign.
• Repeat with the other arm.