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.