When the Tooth Fairy Doesn't Come

Recently my 9 year old lost another tooth. I never realized until we entered the tooth-losing stage of life how often this happens! The next morning I found him under my bed in a foul mood, and when I asked why, he told me the tooth fairy never came. He was heart-broken, and my heart broke a little for him. Thankfully, our tooth-fairy was able to sneak in during the day while he was at school and left a sweet note under his pillow (with a small bonus for her tardiness):

My son found it and said it all made perfect sense now, and that he's never throwing that note away.
Has the tooth fairy ever not shown up at your house? What was her reason?