Maybe Tsuna actually knows why he failed. he just has to think about it. Well Yamamoto did not perform as a rain guardian, and Tsuna knows this, because of Yamamoto worrying to much about losing end tried to give up his carefree self which makes him the most eligible rain guardian for Tsuna, someone who makes them forget the worries of losing or something. That is my opinion.
probably because the two are close friends and due to Vongola`s hyper intuition ,the bossess have,ugetsu expected him to know