How are your supply lines set up? Does each settlement have a Provisioner linking it to the next closest settlement, or do you have hubs for multiple supply lines? How do you know how many supply lines you have running from a particular settlement, and where they originate? Do you dress and arm your Provisioners a certain way, or do you just send them haphazardly into the Wastelands?
After quite a bit of experimentation, I've settled on the "hub" approach; with one hub per 5 settlements. Each hub sends out up to 4 provisioners, all armed and dressed identically, with a weapon renamed for their route(example, Provisioner going from Sanctuary Hills to Red Rocket carries pistol named "Pro SH/RRTS). I also make sure they are well armored and carry a decent weapon, good place to dump extra gear you just can't bring yourself to salvage.
Keep in mind, once assigned to a supply line, the "Settler" label is replaced by "Provisioner", and apparently, that settler remains labeled thusly even if reassigned.
At times, you may find you have a Provisioner you don't need, or who would be better running another route. First off, you'll need to be able to identify him, thus the naming trick with weapons. Secondly, you'll need to find him, which I've found is easiest to do by using the wait command while sitting near the settlement boundary. Just build a chair or bed near where you expect him to come in from. Once he's within your settlement radius, reassign him to some menial task such as farming, you'll then be free to move him to another settlement, fast travel there, and reassign him to a new route.
Hopefully this helps someone, look forward to reading other views.