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.
You might try turning off iCloud for Reminders on both devices, then turn them back on. If want to be OCD about it, turn off, reboot, then turn back on (to clear whatever might be hanging things up).
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.