Since my update from 16.08 to 16.12 it happens often that suddenly a click on a systray icon or a click on the up-arrow of the quicklauncher no longer shows the popup window. E.g. right now I click on the "Networks" icon in systray and what happens is that a blue horizontal line appears above the icon, the arrow icon of the "show hidden icons" changes from up to down but the networks popup itself does not appear. Also clicking on the "show hidden icons" arrow does not show the popup. The up-arrow at the quicklauncher widget currently works, but I also had the issue where just this one did no longer show the popup. To make it work again, I need to logout/in again (or restart plasmashell)
Since I already have my own desktop shell, liquidshell, I don't care any longer.