My last three reminders have failed to come through on my 6S and my Apple Watch so I’m in the same boat. Sorry I can’t help but it’s yet another annoying aspect of Apples recently releases for me.
Last October I installed an update that rinsed my battery in a quarter of the time and left me charging my iPhone by lunchtime. It took Apple 4 months and three iOS updates to fix this. The last update has stopped reminders and killed my battery again! It’s so sloppy and needs to be addressed ASAP.
My reminders have started coming through again since I installed iOS 11.4.1. If yours are still not coming through despite a hard reset, it might be worth speaking to Apple.
I think the problem with asking this here is most posters have newer iPhones than ours and evidently didn’t have this problem. Good luck and I hope you find the fix