Below are the common causes for the tray icon not showing.
- sni-qt is not installed. Try installing it then restart Insync.
- In some cases, when Insync is set to run at startup the tray icon is not shown. To confirm that Insync is running but there's no tray icon, run
ps -ef | grep 'insync start'; if there's an output then Insync is running. In this case, try stopping then restarting Insync to see if the tray icon will show.
- Check if there's an entry in your crontab for Insync by running crontab -l. If there is one try removing it.
- Sometimes an outdated video card driver could cause the tray icon to not show. Try updating your video card driver if possible and restart Insync.
- When using GNOME Shell, legacy tray icons are moved to the bottom left of the screen. They are not shown by default, which makes them hard to find. To show them on the upper right try the TopIcons extension: https://extensions.gnome.org/extension/495/topicons/ (note that in recent versions of GNOME, tray icons have been disabled entirely)
In KDE Plasma 5 the XEmbed protocol was replaced with SNI, which caused legacy tray icons to not be shown. In this case try installing
xembed-sni-proxy. (Thanks to Roman Gaevoy for this tip.)