Unlike on iOS where background sync can fail to work because of system limitations (see iOS support article), Due for macOS should reliably pick up changes made on other devices as long as:
- Due for macOS is running.
- Sync in Background is enabled on your iOS device(s) (Due > Settings > Sync).
If the above are true, but syncing does not happen automatically, there are two possible reasons why automatic syncing can fail on the Mac: One, the background push used to communicate that a change has happened was not sent out; two, the background push was sent out, but was not picked up by the app.
To rule out the first problem, we can leave Due open and running on your Mac and iPhone (unlocked and screen active). Make a change on your Mac, manually trigger a sync (File > Sync Now, or Cmd-Shift-R) and wait for upload to complete.
If the iPhone picks up the changes automatically, then we've determined that the background push needed to trigger the automatic syncing were sent out.
That leaves us with the other possibility: a bug in macOS where the push was successfully delivered to the system, but yet the system did not pass the notification over to Due.
There are some ways to resolve the problem, in increasing order of inconvenience. So perform each step, then try to make a change on your iPhone, trigger a sync in Settings > Sync, wait for upload to complete, and see if the Mac picks up the change. If not, proceed to the next step:
1. Relaunch the Mac app, try again.
2. Unlink from the sync service on Mac app, relink with the sync service, try again.
3. Restart your computer, try again.
4. Delete the Due Mac app, reinstall from App Store, relink with sync service, try again.
If it did not sync (almost) immediately after the changes have been uploaded on your iPhone/iPad, that means it will never pick up the change by itself. So you don't have to wait too long to know that it is not working, and to move on to the next step.