Beds, Food, Water, Defence, Happiness. All in order, OP? Need to have enough room/supplies for everyone, plus newcomers - if any of those things are lacking, you'll have a hard time attracting new Settlers since the existing ones won't be happy.
Also, check Charisma boosting gear - can mess you up sometimes if the game incorrectly reads your CHA at the boosted level for Settlement population purposes - when you take it off, you lose the extra Settlers, and maybe the game permanently reduces your cap in error? This last point is coming mostly straight out of my @ss, so anyone who knows better feel free to give us the real deal on that.
I just reassign Settlers who wander into my undeveloped Settlements (where I can't remove beds completely) - first one becomes the Provisioner for that base back to Sanc, everyone else gets relocated to a place where I actually have stuff and people.
Oh, last thing - try moving your Beacon to the southernmost point of your buiild zone - you cover more of Concord that way, and more roads, so more likely to attract Settlers.