Google Nexus 5 :: Some SMS Reminders From Calendar Not Showing Up

I don't know if this is a problem with the Calendar app, Hangouts, my Nexus 5, AT&T, or Google Calendar on the web. I'm using my Nexus 5 and the built-in Calendar app to sync my phone calendar to the online (web) Google calendar (www.google.com/calendar). For almost all of my events, I've got a Reminder set on the web version to send an SMS to my phone via my standard AT&T, Hangouts-verified phone number. Up until a few days ago, I've gotten these SMS reminders without issue. Suddenly, I'm not getting all of them (I get some, but not all). There doesn't seem to be any rhyme or reason to it. Sometimes they'll arrive, sometimes they won't. Looking at the events in the Calendar show the Reminders are still set up. Rebooting the phone doesn't seem to work?

Also, as I was checking the settings on the phone to see if any of those could be causing the problem, I noticed a setting in the Calendar app for "Pop-up notifications" (Calendar app | 3-dot menu | Settings | "General settings" | Notifications | "Pop-up notification"). I turned it on, but it doesn't seem to do anything (I get no pop-up notifications when one of my Calendar events fire).