I think I recall that when in X, you have to include Shift, e.g. ctrl-alt-shift-F1. Once you're in a console, you don't need the shift.
No shift needed - do ctrl-alt+Fn
... I just tried it using both a usb and old-style (PS2) keyboards.
On Xfce, the initial gui is on F7 (it's probably the same on the other wm.)
If your keyboard has two sets of control and alt (meta) keys, try using each set or a combination. I know this sounds weird, but keyboards issue different scan codes for left and right (at least most of the time