If You get it wrong... wait. (2 seconds) Don't go clickin' all willy-nilly now you hear me! When a match is begotten, wait until the dialogue goes away then do as it commands thee. If only one card disappears when a letter is pressed, press it again er somethin... that seems to work. If not you may have to reset. If a letter does not appear for a match, wait until they both flip back then click them again.
Embrace the Shrek. He will guide you on your journey to success.