Do you have an active (green light) radio tower running?
I assume you have built, and powered, the beacon?
I always fast-travel around a bit, after sleeping for 24 hours, and generally have a couple new settlers arrive for me.
It is a percentage chance for new setllers every 24 hours, if what I have read is correct.
So, maybe the RNG Gods just hate you?
The beacon might not work correctly. I had this problem, today. The switch was set to "ON", the signal showed up in my pipboy, but the message was not playing. I solved it by rapidly switching the beacon on and off, until I could hear the message.
Workshop Mode:
Highlight Settler
Click "Move"
Choose New Location
Storing and replacing the beacon might also be worth a try.
Sometimes the dice gods are not with you. In my game, Abernathy farm went from 6 to 16 in the same time that Sanctuary went from 8 to 11.
As The Fourth Ace posted above, you can move settlers from one settlement to another.
Thanks from me too I am having the same issue as him.
This happened to me once, I didn't know if it was bad RNG or if the beacon wasn't working, so I just turned on the beacons on other settlements and whatever settlers showed up in those were sent to the one where I wanted new ones,
I haven't built any additional beacon in the castle, just using the original radio tower (the one you power up during the quest). So far I got 9 people (1 of them is a settler I moved from another settlement but the rest is "minuteman" named npcs that have arrived). So it should work. Just give it time I guess.
Take Perk: Local Leader
Highlight settler in Workshop Mode
Select "Supply Line"
Choose which settlement to link to.
Two settlements do not have to be directly connected to share resources.
I am bad at explaining this part...but a quick search will give full details, and tips for supply lines.
Yes and no. You need to set up a supply line. To be able to do that you need the first level of the local leader perk. When you have it, target one of your settlers in build mode and hit "Q" ( That is, if you are on PC. Don麓t know which key it is on consoles. ). Then just select the settlement to link to and you are good to go.