I am sure there are multiple servers, for the different countries, as well as for adding new members over time. I wonder if the people who routinely send out requests but get stuck, need a new bunch of neighbors to send to. Perhaps the last few reqisitions are getting "stuck" because a queue of requests is trying to cross multiple servers.
It seems reasonable that someone active but in a totally different time zone, may not see your request for 12+ hours if they go to bed around the time you start playing. And if their accounts are on a different server, it might take some time to cross to those servers & back, especially if in another country.
I too send out to all my neighbors at the same time, including stormy, and haven't had major problems in Castle Story. I don't do small batches. I tend to have these problems only when I am playing, say, Dragon Story, where I do a lot of gifting. That's where my recommendation to scroll to the bottom of the Gift queue comes from, I send a gift from 7 days ago and a requisition might immediately pop up! That wasn't there when I opened the game.
It won't help for this event, but maybe for future ones, consider adding some Forum members (who are accepting neighbors, that is) who post online around the same time(s) you do, will get you quicker responses in future events. I started doing this a few years ago. I know I am slowly weeding out neighbors from my accounts, I have turned off Auto Join, and am only accepting/keeping new members from the Forum (plus those who post occasionally on my wall, I Know they are active haha) And I am not having issues with requisitions in Castle Story (keeps fingers crossed)