I did an update from 18.04->18.10->19.04 Ubuntu. After the update the switch applications or switch windows functionality is not working. Tried alt-tab and super-tab.
I tried to reset the keyboard shortcut in settings and tried again after setting it. still the same. All the other shortcuts with alt works. For ex: alt-f4 close the window. But when I replaced alt-f4 with alt-tab it didn't work.
I had same problem.
It seems that in Ubuntu 19.04 (or maybe from 18.10) the "switch applications" changed shortcut key combination.
Now it is "super+tab".
If you want to change to alt+tab just go to:
Settings > Devices > Keyboard and under Keyboard Shortcuts change Switch applications shortcut