I barely got into NPCs and I have problems into creating them.
You aren't supplying much information, so I'll assume that the the arms are CanCollide false and the NPC has no joints.
Use :MakeJoints() on the NPC Model to create joints for it, which should stop the arms falling off.
just create a script inside the arms and inside it put this:
1 | local function anchorNPC --Name this anything you want |
2 | script.Parent.Anchored = true |
3 | end |
EASIER WAY:
1 | script.Parent.Anchored = true |