Alma 21

If missionaries become discouraged because of limited success, they should read the record of Aaron after he and Ammon had seperated. Though Ammon seemed to have immediately successful, we learn that Aaron had -

"...fallen into the hands of a more hardened and a more stiffnecked people; therefore they would not hearken unto their words..." (Book of Mormon | Alma 20:30)

But yet, Aaron was diligent in his efforts. For those discouraged missionaries, they should read the 21st chapter of Aaron where -

"...Aaron came to the city of Jerusalem, and first began to preach to the Amalekites. And he began to preach to them in their synagogues, for they had built synagogues after the order of the Nehors; for many of the Amalekites and the Amulonites were after the order of the Nehors." (Book of Mormon | Alma 21:4)

After some discussion and contention amongst the Amalekites -

"...Aaron began to open the scriptures unto them concerning the coming of Christ, and also concerning the resurrection of the dead, and that there could be no redemption for mankind save it were through the death and sufferings of Christ, and the atonement of his blood." (Book of Mormon | Alma 21:9)

It is obvious that Aaron taught and bore testimony of the truth, but even so -

"...they were angry with him, and began to mock him; and they would not hear the words which he spake." (Book of Mormon | Alma 21:10)

After much faithful effort, Ammon -

"...saw that they would not hear his words, he departed out of their synagogue..." (Book of Mormon | Alma 21:11)

He was then rejected by the people of Ani-Anti. After that he was rejected in the land of Middoni and was cast into prison. Who wouldn't become discouraged. But after being released from prison after much suffering, he did not give up for he -

"...went forth again to declare the word, and thus they were delivered for the first time out of prison; and thus they had suffered.
16 And they went forth whithersoever they were led by the Spirit of the Lord, preaching the word of God in every synagogue of the Amalekites, or in every assembly of the Lamanites where they could be admitted." (Book of Mormon | Alma 21:15 - 16)

It was after this faithfulness that he had great success even with the very king of all the Lamanites.