I'm glad it's not just me having this problem.
I set up Greentop Nursery (or whatever the name is) and assigned two settlers to supply lines; then there was an attack where the remaining settlers were killed. Now the settlement has 0 food production because both living settlers are off carrying supplies, and can't be reassigned to farming; while no new settlers are appearing. It's been several game weeks, and NO new settlers have come to the beacon.
It seems there is a max number of settlers that will come to each settlement- but I don't know how to tell what that number is.
It also seems to reveal a really depressing view of the world, since raiders, super mutants, and other enemies spawn endlessly, but settlers have a finite number. As if there are only a few people in the world who are willing to work for a living, and all the rest are just going to steal from them. How very, very cynical of Bethesda, if they did in fact set up the game this way.