The Rosh wrote (Pesachim Chapter 10, Siman 20): "One needs the reason for the matzah in the size of an olive for the first one and the size of the afikomen, and if one ate without the reason, he did not fulfill his obligation, as Rabbi Yehoshua ben Levi said: If one ate an olive size of matzah while reclining, he fulfilled his obligation, implying, while reclining yes, without reclining, no, and if one ate without the reason, he must go back and eat it with the reason." This means that it is clarified in the Gemara that one fulfills his obligation only if he reclines while eating, implying that if he does not recline, he did not fulfill his obligation and needs to go back and eat it reclining. However, the Rambam (Chapter 7, Laws of Chametz and Matzah, Halacha 8) ruled differently, stating that the reason is needed during the eating of an olive size of matzah and drinking the four cups, and for the rest of his eating and drinking, if he reclined, it is praiseworthy, and if not, it is not necessary. He did not write that if one did not eat with the reason, he should go back and eat with the reason. It requires clarification, as the Rosh brought an explicit Gemara that one fulfills his obligation only if he reclines, implying that if one eats without the reason, he did not fulfill his obligation and needs to go back and eat with the reason. Why did the Rambam omit this rule of going back and eating with the reason?