I've seen nice "loops" of supply lines on the net.
But in my game, I often cannot select nearby settlements as targets in the "supply line target selection list" when setting up a supply line, because they are greyed out.
The further I progress in the game the more confusing does the supply line target selection become. It seems to get smaller and smaller.
e.g. in Nordhagen Beach I cannot select Finch Farm nor County Crossing.
Why is that? Can I influence this by changing other supply lines?
EDIT1: Seems like I have to clarify my personal game state here:
I have the local leader perk. I have many settlements, which I usually build by building a recruitment bacon and assigning the first settler which walks into the newly aquired settlement to be provisioner. This worked fine for the first 10 settlements. The issue came up only recently, namely on the settlements of Nordhagen Beach and Somerville Place.
I do not intend to build a supply line hub, I just want to connect the settlements to the nearest settlement in the already existing network (which is not a closed circle, no has it a central hub)
EDIT2: Here is a screenshot of my current supply lines and the target selection list, viewed in Finch Farm. It's not showing Finch Farm being disabled, because I have no savegame from before setting up the supply line and later savegames (after I already created a line to a different settlement) seem to have fixed themself.
Best Answer
I think I found a reason.
All those settlements which are disabled in the target list have something in common: They all report that they have reached or exceeded my current maximum of settlers, which is 19.
Two of the disabled settlements even report ridiculous 31 and 37 settlers. I did some tests with my settlements. Below are my findings.
The problem can be solved by reducing the amount of settlers 1 below your current max amount of settlers per settlement. That means, 10 plus [current charisma with all boosts] minus 1 (see this question for calculation the maximum amount of settlers per settlement)
So the solution is pretty simple: Send away settlers to under-populated settlements (e.g. your new one!)
Reaching or exceeding the maximum does disable this settlement in the "supply line targeting list". However, the provisioner do not count against the target settlement's counter, even when the above rule seem to suggest just that.
That might not make any sense, but it seems to fix the problem.