It seems to me that I get only ONE chance to assign a settler to a specific task. That's when I am at the settlement and meet them for the very first time and they ask "Is there anything you want me to do?" Anything other than "Yes" and there goes that one chance.
So it makes me wonder: What are all the other settlers doing that I did NOT assign? How do they decide which tasks they will self-assign? Do they NOT do anything other than scavenge? (In an area that I have already scavenged clean before they ever arrived.) Is there any way to force any settler to do the task I assign regardless of what they were doing before? It seems like the ONLY topic I get with any settler after the assignment window has closed is to trade Inventory items.