There's a problem with the request system everyone should know about and plan around accordingly.
We had many complaints on the forum before, when neighbors didn't receive requests when they should.
One of the reasons is some of the requests get discarded by the game.
If you send a request to a neighbor and this neighbor get 10 more requests before answering yours, your request will be deleted forever.
There's no way to prevent it unless you know when your neighbors are playing and checking their news.
Even if someone just visited or sent a request to you, they might have already left by the time you sent your request.
All you really can do is to have plenty active neighbors, don't rely on just "20 active neighbors" as previously believed.
And of course, try to know your neighbors. If you know that most of them live in USA, especially East coast, then maybe it's not always a good idea to sent requests right after S8 day reset (midnight), as we thought before.
At the same time, someone could be busy during the day. There's no easy solution. It's pretty easy to get 10 requests even in a few hours for an old player with active neighbors. I guess that many of us have 100-200(?) neighbors.
Luckily, S8 acknowledged there's a problem and decreased the requirement for the events to 15 requests, so it's realistic for everyone to meet this goal now. Hopefully, they'll fix this glitch soon.