From memory.. I think Sync syncs metadata only. You need to press Sync Settings.Select the image you want to copy edits from. The select the images you want to past the edits to using CMD-Click.
Then click "Sync" at the bottom of the slides panel.
A dialog will appear asking you to check which edits you want to paste.
Syncs metadata in Library mode, develop settings in Develop mode.From memory.. I think Sync syncs metadata only. You need to press Sync Settings.