Here are some possible issues:
- An incorrect date/time on the Mac/mobile devices
- An incorrect date/time is known to cause sync issues.
- An outdated client
- If the mobile client is outdated and the desktop client is up-to-date, it's possible that the mobile client doesn't understand how to decrypt notes encrypted on desktop. See also If you have a problem with encryption, please read this first! · Issue #8446 · laurent22/joplin · GitHub
On mobile, the "sync status" screen (under "Tools" in settings) might give additional information about the issue.