1. Your question is asked by the Yad David: How is it possible that there was not a single person present who was capable of reminding Shmuel ha'Katan of the text of the Berachah?

2. The Me'iri writes that since the Nusach of the Berachah was "Mechudash" -- it was a new and unfamiliar text -- other people apart from Shmuel ha'Katan did not yet know it.

3. Let us explain futher the words of the Me'iri. How could it be that only Shmuel ha'Katan knew the "Nusach Mechudash"? Even given the fact that it was a new prayer, it still seems from the Gemara that it was a year after the original composition of the prayer that it was forgotten by Shmuel ha'Katan, so how -- in that span of time -- had others not learned the prayer?

4. We must realise that it was not permitted in the time of the Gemara to commit these prayers to writing. These is learned from the Gemara in Shabbos (115b) that if there is a fire on Shabbos one may not save written blessings. This is because it is forbidden to write the blessings down and the Halachah states that anything which one may not write down, one may not save from a fire on Shabbos. The reason why it was forbidden in the time of the Gemara to write these blessings is that they are not considered part of the Written Torah but rather the Oral Torah. There is a Halachah (Gitin 60b) that one is not allowed to write down the Oral Torah. (It is is permitted to write the Oral Torah nowadays because, as the Gemara in Berachos 63a bases on the verse of "Es la'Asos la'Shem...", there would be a danger of forgetting the Oral Torah due to the difficulty of the exile and the weakening of people's memories.)

5. It appears to me that there may be another reason why only Shmuel ha'Katan knew the Birkas ha'Minim. This is because it takes time for a Gezeirah of Chazal to come into widespread practice. This is apparent from the Gemara in Avodah Zarah (36a) which relates that Chazal originally instituted a Gezeirah that prohibited the consumption of oil of Nochrim. However, Chazal checked the progress of this decree and discovered that the majority of the Jewish people had not accepted this prohibition upon themselves. Chazal then relied on the teaching that one cannot make a Gezeirah on Klal Yisroel unless most of the people are able to abide by it. We learn from there that it takes time before an institution of Chazal acquires popular acceptance.

Since, in the times of Shmuel ha'Katan, only the prayer leader recited Birkas ha'Minim, and especially if we assume that in the place where Shmuel ha'Katan lived he was the regular Shali'ach Tzibur, it is reasonable that indeed the blessing did not enjoy widespread popularity yet and thus no one else was able to prompt Shmuel ha'Katan when he forget the text of the blessing. (See the Chidushei Maharam Benet on Berachos, by Rav Mordechai Benet, a contemporary of the Chasam Sofer, who writes that the reason why the Mishnah on 28b mentions only 18 Berachos in the Shemoneh Esreh, even though the Mishnah was written after Raban Gamliel who instituted the 19th Berachah, is that at that time only the Shali'ach Tzibur said Birkas ha'Minim.)

Kol Tuv,

Dovid Bloom