Some key combinations that may work on standard keyboards may not be recognized on Apple's portable computers. This includes combinations containing more than four keys and some combinations of three keys across the home row (asdfghjkl;').
This is done to avoid an issue with modern keyboards called "phantom keys". Phantom keys occur when the controller reports a key to be down that is not really down. To avoid this, the controller will stop sending keycodes if key combinations are pressed simultaneously that can cause this condition—combinations like more than four keys or some combinations of three home-row keys.
If you require the use of combinations that are not possible with the Apple portable computers' built-in keyboard, you can connect an external keyboard to the computer. Since the matrix used on external keyboards is different, they will allow different key combinations that aren't available on built-in keyboards.