Not sure why, but there seems to be a lot of issues with gnome and app indicators (think system tray)..
So far this seems to work, but as it is a bit of voodoo, haven't tested it often.. (or at least some combination involving the following):
$ sudo apt purge gnome-shell-extension-appindicator indicator-common indicator-application
reboot / Alt-F2 'r'
gnome-tweaks -> extensions. make sure everything is disabled
reboot / Alt-F2 'r'
Ubuntu Software -> Add-Ons -> Shell Extensions -> Topicons plus (install)
reboot /Alt-F2 'r'
gnome-tweaks -> extensions make sure Ubuntu Dock (optional) and Topicons plus are enabled
reboot / Alt-F2 'r'
Rebooting at lot is excessive and isn't necessary, but pointing out that a reboot at some point is a good test to make sure changes are actually taken.
--Doug