I'm using ui select2 material in angularjs with buttons next and previous

when i click on previous or next button the data changed as i want, but the title in the select box never change.

<span class="forward-icon-left" ng-disabled="loading" ng-click="previousUser(selectedUser)" ng-if="submittedPollUsers.length > 1"></span>
                    <select ui-select2="select2Options" class="section-selector rounded-corners"
                            ng-model="selectedUser" 
                            data-wait-for="submittedPollUsers" 
                            data-ng-if="submittedPollUsers" 
                            ng-change="changeUser(selectedUser)"
                            ng-options="usr.localizedName for usr in submittedPollUsers track by usr.userHashId">
                    </select>
                    <span class="forward-icon" ng-click="nextUser(selectedUser)" ng-if="submittedPollUsers.length > 1" ng-disabled="loading"></span>