It is good to get into the habit of syncing your library periodically. Syncing will update your changes to the cloud so that you are able to view them in the web and desktop versions of Mendeley. If you are connected to the internet, your library will automatically sync from time to time, but you can also sync manually.
To sync your library, select on the grey sync icon in the bottom left corner of the black sidebar in your Mendeley library. The icon is two grey arrows that form a circle. If you hover over the sync icon, it will tell you the last time your library was synced.
