Changes don't appear on another device. What should I check?

Check these in order:

1. Internet on both devices. A change reaches the server only when the device that made it is online. A plan changed in the desktop app without internet shows "not sent yet" and goes to the server by itself when the internet is back.
2. The same account and church. Make sure both devices show the same church.
3. Refresh. In the browser, reload the page. On iPhone and iPad, tap Sync in the Songs tab, and Settings → Songs → Last sync shows when it last happened. On Android, pull the list down. In the desktop app, look at the bar at the bottom: Online · live sync means changes arrive within a second, and if you see Offline, click it to try connecting again.
4. Permission to save. In the desktop app, without the Service plans: Edit permission, changes to the plan stay on that computer.
5. Device settings. Chord notation, text size, the theme and pedal settings belong to each device, so they don't move between devices.
6. The app version. Update the app. New versions come to Mac first, so Windows and Linux can be behind.

Still nothing? Check Known issues, then post in Bug reports following How to report a bug.