The keyboard is not responding or letters are being skipped during the exercise

First of all: click on the exercise text to make sure the window has focus. If you have switched tabs or clicked outside, the keystrokes do not reach the exercise. This is, by far, the most common cause.

If it keeps happening

• Check caps lock: the application warns you if it is activated, and with it, everything you type is considered an error.
• Check that your keyboard layout matches your keyboard. Look at Settings → Keyboard layout: if it doesn’t match the keyboard in front of you, there are keys that won’t produce the expected character — and it’s usually a punctuation key that gives it away first.
• Disable browser extensions, especially grammar checkers and translators: they intercept keystrokes.
• Try incognito mode to rule them all out at once.

If letters are skipped when typing fast, it is usually the physical keyboard itself: cheap membrane keyboards miss keystrokes when typing at speed. This is more noticeable just as you start to go fast, and it is not a Kluppy issue: try it in a word processor and you will see the same behavior.

Programs that type for you do not work: the application only accepts real keystrokes, because otherwise the scores would be worthless.