I have Messages in iCloud on my iPhone (6S, latest OS version 11.4) - or at least I think I do: In iPhone Settings -> [my Apple profile] -> iCloud I have Messages switched to on. I also have Messages on my MacBook Pro, which has the latest version of Mac OS.
However today my iPhone was off because it didn't have enough charge. My Mom (who also has an iPhone with iMessage) sent me a message. The message showed up on my Mac. However even after I restored power to my iPhone and restarted it a couple times, the message never showed up on my iPhone.
So what is going wrong? I thought this, at least, would be solved by Messages in iCloud.