Fortunately, Hermione stuck to his side like glue, and she had an eye for detail. After she got the spell working for herself (and she was always the first to get it working), she would turn to Harry and point out the little wobble he was introducing in the flick, or the way he was slurring the third syllable, or how his concentration seemed to waver when the matchstick started to shift. He wasn't always the second to get the spell, but he always got it, and that's what counted.