Reminders not syncing across devices

When I make an update to reminders in my browser, it updates across my other devices. If I make a change on my iPhone, it does not update anywhere else. (I also have a recently purchased Macbook that is a hot mess and nothing works right over there. Right now I'm just trying to fix the phone issue.)

Everything is updated. None of my other apps are affected. Calendars, notes, etc., work exactly as they should.

What do I try next? I'm followed all the available help articles so far with no improvement. Thanks!