I've upgraded kubuntu 19.04 to 19.10.
During 19.04, animation desktop with a cube when switching effect worked well. The time for switching animation was set to 2000ms.
But after upgrading to 19.10, it doesn't work at all. Switching is completed immediately every time.
I checked 'KDE System Settings'.
'KDE System Settings'>'Virtual Desktops'>'Options'>'Animate desktop switching with a cube' Check box is checked and 'Desktop Cube Animation' is selected.
I installed the compiz package but it doesn't start on Plazma desktop.
What should I do to animate when switching virtual desktops?
Found a solution by myself.
I don't know why Compositor forgot the status of 'Enable Compositor on startup' check box, but this procedure resolved the issue.