Some people dissented from Rabbi Ami and said that in his decision, he acted for personal reasons. What about the rule that Rabbi Ami quoted? They said that rule applied only to drought, but they could keep fasting for other afflictions until they were answered.
So, did Rabbi Ami simply make a mistake? - No! There were two opinions on this in the earlier times. Rabbi Yehudah the Prince reasoned that they should not burden the community with more than they could bear. The opposing opinion said that it only applied to drought because after the time it took to go through thirteen fasts, it was already spring, and the rains would not be beneficial anyway. That is why they would stop fasting. The final, practical conclusion follows this opposing opinion. (Still, today, there are no courts that declare congregational fasts.)
Art: The Court by Jean-Louis Forain (1852-1931)