Subtle
Harry's flirting was subtle. A wink so quick it could be a blink, a flush of red across his cheeks upon eye contact, a brush of fingertips on the back of her hand. Small, tiny things that had happened for years. So understandably it was difficult for Hermione to realise that Harry was trying to tell her something.
But when she finally got it, when she finally grasped that yes, he liked her, it was all worth it.