Q: Perpetual issue syncing Reminders

I have two iPhones, a Macbook and an iPad Air. I am having a perpetual problem with syncing Reminders across devices. Now, I've read through a lot of these forums, and I've tried solutions like this one - Reminders not syncing across devices . They work for a time, but then the problem comes back eventually.

It's one of the iPhones - a 6S - that doesn't sync. The other one, a 6, does. All their settings are the same, all the iCloud syncing settings and all that. Both have the latest iOS. The only difference is that one's a 6S and one's a 6. (The only other difference is that the 6S is on AT&T and the 6 is on Verizon, but this is happening over wifi, so I'm not sure the carrier makes a difference.)

Situation: I'll create, say, a grocery list on my Macbook. It will immediately show up on my 6. It will not show up on my 6S until ... sometime later, if at all. Sometimes, after repeatedly opening and closing Reminders it will finally sync. But why does the 6 do it immediately and the 6S not?

I missed an alert today because I only had my 6S on me; it hadn't synced the Reminders at all. I only saw it when I got to my 6 and there was the reminder in the notifications screen.

Again, I've done the iCloud sync "fixes" like in the above, and that works, temporarily. But this is a continuous issue. Any ideas?