The year interval between the first Nephite initiative and this engagement at Teancum suggests that Desolation’s defenses were inadequate when the retreating army tried to make a stand there against the rapidly advancing Lamanite/ Gadianton army. Desolation was the first city north of the narrow pass.
There was probably a pause between the conquest of Desolation and the attack on Teancum to allow for planting. When hostilities resumed, the Nephites had had enough time to strengthen Teancum’s defenses. Their stand there was successful, almost certainly benefiting from the respite that allowed recovery. Perhaps Teancum was also a larger, better-fortified city.
The Lamanite supplies had to come north through the narrow neck, suggesting a long supply column subject to delays. And perhaps, if planting season had just finished, supplies from the previous year’s harvest were in short supply. Thus, the Nephites succeeded in retaking Desolation. Although Mormon gives few details, if my hypothesis about the supply lines is correct, the Lamanites may have beat a strategic retreat, abandoning Desolation without a staunch resistance.