I have this issue too with my iPhone, iPad and MacBook Air. When I create a reminder on my iPhone for example, it won't sync to my other devices unless I open the app individually on all my other devices. I don't have a fix for it though.
Not sure when or how but my devices started syncing a few days after my post on March 2 above. It’s been working flawlessly since. Again I don’t know if it was on my end or on apple’s or something else altogether.