iCloud Drive does not sync to iPhone

iCloud Drive syncs files and folders between my iMac and my MacBook Pro, and also shows up on my two iPads. But it appears empty on my iPhone. I've tried everything I can think of. Re-botting the phone, turning iCloud Drive off and then on. Signing out and signing back in.

Again, iCloud Drive shows up perfectly on all my other devices. But not on my iPhone. Any ideas why this is happening and how I can fix it?