In this chapter Mormon tells at length the following episode: Using both a stratagem and a former Lamanite, a man named Laman (one of the servants falsely accused of killing the Lamanite king), the Nephite prisoners in the city of Gid are liberated (55:4–26).
Moroni would not accept Ammoron’s terms—“Behold, I will not exchange prisoners with Ammoron save he will withdraw his purpose”—because he did not want to give Ammoron any more power than he already had. Nor would he take the “new terms” that Ammoron was suggesting, which was to surrender to the Lamanites and allow them to exercise what they regarded as their right of government. Ammoron accepted the prisoner exchange deal that was clearly in his best interest, but Moroni was not willing to reciprocate unless Ammoron withdrew his purposes, which Moroni considered fraudulent (see v. 1).
Book of Mormon Central, “Why Did Moroni Change His Mind about Exchanging Prisoners with Ammoron? (Alma 55:2),” KnoWhy 163 (August 11, 2016).