Why won't the back of my shirt stay tucked in?

Each BLFAC group tours Europe for 3-4 weeks after an intensive week of rehearsals at Blue Lake. Each group tours 2-4 countries (France and Germany are the most frequent), staying in a total of about 7 cities. The groups stay with European host families for about 3 days each, providing them with a unique first-hand cultural experience. During their stay in each city, the groups give a performance featuring a variety of musical styles. After their tour of Europe, the groups return home for a welcome back concert.