Is the Imam’s recitation sufficient for every follower?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.
The issue of reciting Al-Fatihah after the Imam in a congregational prayer whether such a prayer is performed loudly or silently is a controversial issue. Some see that the recitation of the Imam is sufficient for followers depending on what is said: “The Imam is appointed only to be followed.” Others see that the followers in a congregational prayer should recite it.
Having examined what scholars said, we can conclude that in the loud prayers, it is better to listen attentively to the recitation of the Imam. If the prayer is performed silently (Dhuhr and Asr prayers), a follower should recite Al-Fatihah and other verses from other chapters.
Moreover, if you want to know whether you should make up the previous prayers or not, I will tell you that you do not need to. That is because of two reasons. 
First, if it is supposed that you made a mistake, you do not need to make prayers up because you had not known the appropriate rule. It is narrated that the prophet had said: “Allah pardoned my nation’s errors.” Thus, you are not sinful. 
Second, as long as there is no one final rule on this issue, a Muslim can adopt any opinion regarding the recitation of Al-Fatihah in a congregational prayer. 
It is still a controversial issue; each group of scholars has their considerable evidences. We cannot consider one opinion better than the other.