In this base, for example, if my outer gates are closed then campaigns will sometimes fail to properly approach my base. They will attempt to path to the innermost gate of the arena (the square with tier 3 walls), and unless my front gates are open and allow them to do that, they will simply wait outside my base without setting off the campaign event. Dismantling those gates fixes the problem immediately.
The town center sits around the red circled X. All of the gates, walls, and buildings are set to private, as is the town. I've seen this problem occur with multiple such nested wall structures. It seems the NPCs aren't satisfied unless they reach the deepest gate, whereas my base is designed to stop them at the outermost.
I'm happy to upload a save but I'm running some mods, so I'm not sure how useful it would be. I could probably repro this issue unmodded since I've seen it over the span of multiple saves and bases.