In the Gemara in Rosh Hashanah (19b) it says:
והאמר רבי חיננא בר כהנא א"ר מימות עזרא ואילך לא מצינו אלול מעובר לא מצינו דלא איצטריך הא איצטריך מעברינן ליה
Didn’t Rabbi Chinnana bar Kahana say that Rav said: From the days of Ezra and onward, we have never found that the month of Elul had an additional day. We have not found that the month of Elul ever had an additional day, because it was not necessary to add a day. But if it had been necessary, they would have added an additional day.
But then one daf later it says (20a):
כי אתא עולא אמר עברוה לאלול אמר עולא ידעי חברין בבלאי מאי טיבותא עבדינן בהדייהו
When Ulla came from Eretz Yisrael to Babylonia, he said: This year they added an extra day to the month of Elul. Ulla continued and said: Do our Babylonian colleagues understand what benefit we did for them?
Which is it? If they never needed to add a day to Elul, what is Ulla saying? And similarly if they did, what is Rabbi Chinnana bar Kahana saying?