You need to make sure you're saving changes back to the original file within the Zotero data directory and that the file's timestamp is being updated when you save.
Note also that linked files (stored outside the Zotero data directory) aren't synced by Zotero. If you're using linked files, you'll need to sync the files yourself or switch to stored files.
Upgrade Storage
Note also that linked files (stored outside the Zotero data directory) aren't synced by Zotero. If you're using linked files, you'll need to sync the files yourself or switch to stored files.
For general file sync issues, see Files Not Syncing.