With regard to the worshipper reciting al-Faatihah when he is praying behind the imam, this is one of the issues concerning which there was a difference of opinion among the scholars (may Allah have mercy on them).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The (scholarly) opinions boil down to three: two opposite views and one in the middle. One of the two opposite views is the view that he should not recite behind the imam in any circumstances. The second view is that he should recite behind the imam in all circumstances. The third view, which is the view of the majority of the early generations, is that if he can hear the recitation of the imam, he should listen attentively and not recite.
End quote from Majmoo al-Fataawa, 23/265
The opinion followed on this website is that reciting al-Faatihah is obligatory for the one who is praying behind an imam both in prayers in which the recitation is done quietly and those in which it is done out loud. This has been explained in the answer to question no. 10995
If a person does not recite al-Faatihah following the opinion of a scholar who says that it is not obligatory, or he does not do so because he is unaware that it is obligatory, there is no blame on him and his prayer is valid.
Shaykh Ibn Baaz (may Allah have mercy on him) said: If he does not recite it - i.e., al-Faatihah - because he thought that it was only required of one who is praying on his own or the imam, or because he was unaware of the shari ruling, then his prayer is valid, because he did not deliberately do something that Allah has forbidden or omit something that Allah has made obligatory; rather he did not do it either on the basis of what he thought was correct or out of ignorance of the shari ruling. So in this case his prayer is valid. But in the case of one who was aware of the shari ruling and believed that it was obligatory upon him, but he omitted it deliberately, this renders his prayer invalid, because he went against what he believed and he went against what he knew to be the truth.
End quote from Fataawa Noor ala ad-Darb, 12/344
Based on that, you do not have to make up the past prayers, because they were valid.
And Allah knows best.
Reproduced from Islam QA