What the title says, I have eight workspaces separated in two rows with four columns each. I can change by clicking with the mouse, but the Ctl+Alt+Left/Right arrow keys doesn't work.
Any suggestions?

Thanks in advance!

Solution:
Had to go to System Settings -> Shortcuts -> Global Shortcuts -> KWin. There I located the "Switch to Desktop x" actions (defaults were Ctl+F1 for Desktop 1, etc.).