I've come across quite the bug.
I've been unable to ally with the following settlements: Nordhagen Beach, Oberland Station, and County Crossing. Preston nor Radio Freedom will give me any quests related to these areas so that I'm able to ally with them and use their workshops. Just a few other radiant quests to save settlers from kidnappers, etc, and it seems I get quests for Tenpines Bluff more than anything. Ugh.
After scouring Google about this issue, it seems a lot of others have had the same issue. Several theories like if you don't ally with all the settlements before level 50, you're out of luck... all saying this has been happening since patch 1.5.
I'm aware that I could use the console to "setownership" of the workshop, but that comes with a lot of issues/bugs. And I know that I could simply slaughter all of the current settlers at those locations to obtain ownership. But I'd really rather do neither. Nordhagen Beach has a child settler, for goodness sake!
Anyway, I'm hoping it would be possible to create a holotape or something that would allow you to select quests to force. For instance, a "Kidnapping at Oberland Station" radiant quest so that I'd be able to complete it and become an ally to the settlement.
Or even just a bug fix???
Thanks for checking out my mod request. If anyone has any insight to this problem or has had this problem themselves and would also be interested in a fix, please leave a comment.