Just copying what Lswest wrote more than two years ago: if your TTY switching is not working, and you have a custom ~/.Xmodmap, make sure your mappings for the function keys in it are as follows:
keycode 67 = F1 F1 F1 F1 F1 F1 XF86Switch_VT_1
keycode 68 = F2 F2 F2 F2 F2 F2 XF86Switch_VT_2
keycode 69 = F3 F3 F3 F3 F3 F3 XF86Switch_VT_3
keycode 70 = F4 F4 F4 F4 F4 F4 XF86Switch_VT_4
keycode 71 = F5 F5 F5 F5 F5 F5 XF86Switch_VT_5
keycode 72 = F6 F6 F6 F6 F6 F6 XF86Switch_VT_6
keycode 73 = F7 F7 F7 F7 F7 F7 XF86Switch_VT_7
keycode 74 = F8 F8 F8 F8 F8 F8 XF86Switch_VT_8
keycode 75 = F9 F9 F9 F9 F9 F9 XF86Switch_VT_9
(And, in case Ctrl + Alt + Backspace also doesn’t work:
keycode 22 = BackSpace Terminate_Server BackSpace Terminate_Server BackSpace
but you’ll also need to have it enabled in Keyboard Layout Options, under “Key sequence to kill the X server”.)