The photo you want to sync from must be the Most Selected, and the photos you want to sync to must be selected.
When you activate "Auro sync" nothing happens instantly, with the files selected any further edits you do will be applied to all the selected photos. Hint, make sure you turn it off when you are through. If not the next time you select multiple files "Auto sync" will become active again.
Be aware "Sync" and "Auto sync" function differently.