Are you asking them directly, or is the principal somehow involved? If the principal is not involved in this at all, I would ask her/him to make the appeal to the teachers.

We're having an issue at our elementary school with teachers not volunteering to proctor our "fun nights" for students. School policy dictates that school events have to have between 2-4 teachers there. Some teachers have been asked to be paid to attend, which we do not want to do since the money we are raising goes to them and their classes. We pay for field trips, classroom activities, assemblies etc. and we give them an amazing Teacher Appreciation week every year where they all receive gifts such as gift cards, Kindles, YMCA memberships etc. How can I convey to them how important it is that they participate? We're only asking for 2 hours of their time and only one time per school year. It's very disheartening.