I have three pairs of AirPods and have used them on a handful of iPhones now, I always have the same issue. That is when you take them out, one will work and the other will not, you have to put it back in the case for it to work. So many times I've left the house to walk the dogs, left the case at home and then put on music to discover it isn't working. Now I've asked so many people in real life who own them and they've all said it happens to them too. I've asked around on the internet and I get lots of people saying it happens to them.
Seems to be a major flaw that no one ever talks about.
I also occasionally have only one work, but I would be happy if all it took was putting them back in the case for them to work.
I have had to repeatedly put in case, take out of case, curse, reboot my iPhone, reboot my Apple Watch, restart the bluetooth connection, put in case, out of case, curse, reboot iPhone, reboot Apple Watch, restart the music, put in case, take out of case,,,,,, etc etc etc, you get the picture.
20 minutes later it is working again and I can't figure out what was the magic sequence that got it working again.
Fortunately this only happens every few months, but is completely unpredictable and unrepeatable