It does take time to attract settlers. They don't show up en-mass. I am currently maxing out at around 17-18 settlers and when I turn on a beacon at a new settlement it can take maybe an in-game month or two to get from 0 to 17-18, so sometimes they show up quickly, other times they trickle in. Your experience doesn't sound bugged to me, just be patient. Not sure you can do anything to speed it up.
Also, I know there are a few signal boosters around the commonwealth, but they are not something you can build, you just need to find them and turn them on. Not sure they'll help with settlers though, I think they are for a quest or something (they might help).
Also, I have a bug in my game (and so do others) that when I fast travel away from within the range of one of my settlement recruitment beacons my pip-boy shows the wrong number of settlers, or 0 water, or not enough beds, or any other number of wrong things in relation to settlements. These are wrong of course, since when I travel back there is plenty of water, and beds, and the number of settlers is correct, but as long as I am away the settlement(s) are losing happiness due to the wrongly reported numbers on my pip-boy. So now I have to hoof it far enough away from any recruitment beacons before fast traveling. Just FYI. Something to look forward to. 