Question: Why can't I find all of the collects and postcommunion prayers you say in my missal?
Answer: In all cases, we are talking about additional prayers added after the collect, secret, and postcommunion prayer of the day in low Masses. The set of three is often referred to as "the orations." Until they were made optional by Pope Pius XII in 1955, the additional prayers in question were indicated in hand missals used by the people as well as the altar missal used by the priest. Prior to that, certain prayers, which varied by season, were required on vigils, Masses of the season, and in the Saturday Masses of the Blessed Virgin. After 1955 these prayers could be said or not, at the discretion of the local bishop, or if the bishop made no rule, at the discretion of the priest.1 They are still printed in missals, in a section containing the votive prayers for a variety of intentions. We normally mention the prayers that will be used in the "Ordo" section of this Bulletin.
Even after 1955, some feast days had additional prayers to commemorate a saint or saints not the primary saint of the day. (For example, on July 25th, the Feast of Saint James, a set of extra prayers is included in the Mass to commemorate Saint Christopher.) A similar commemoration - of Advent and Lent - takes place in Masses of the Saints occurring during those seasons. In the Masses of the day during Lent there is an additional prayer "over the people," said after the last postcommunion prayer.
Additionally, the Roman Missal contains a number of votive prayers which the priest may include for various intentions on many weekdays throughout the year. There are prayers for the dead, for the sick, for rain, for peace and in time of war, for the Pope and for the Church, for the civil authorities, and so forth. Many of these prayers can be found in a section of your hand missal. If you have trouble finding them, ask Father for help.
1. Decree "Cum nostra hac aetate," 23 March 1955.