<apex:page StandardController="User" Extensions="UserData" tabstyle="User" sidebar="false">
  <apex:form >
  <apex:pageBlock title="Select Users" id="USERSLIST" mode="edit">
    <h1>Update Sales Co-ordinator for the users in Particular Region</h1>
  <br></br>
  <br></br>
<apex:actionRegion >
<apex:outputLabel value="Select Region of the user: " for="viewlist" />
<apex:inputField id="startMode" value="{!User.Region__c}">
<apex:actionsupport event="onchange" action="{!doSearch}" rerender="theTable" />
</apex:inputfield>
</apex:actionRegion>
           <br></br>
   <apex:pageBlockTable value="{!results}" var="t" id="theTable">
        <apex:column >
          <apex:inputCheckbox value="{!User.ManagerId}" id="checkedone">
          </apex:inputCheckbox>
        </apex:column>
        <apex:column value="{!t.Name}"/>
        <apex:column value="{!t.Email}"/>
        <apex:column value="{!t.Region__c}"/>
        <apex:column value="{!t.Manager__c}"/>     
  </apex:pageBlockTable>
<apex:inputField value="{!User.ManagerId}" />

<!--THE BELOW MENTIONED COMMAND BUTTON ACTION IS NOT GETTING INVOKED--->
<apex:commandButton value="UpdateManager" Id="Update" action="{!getselected}" />
    </apex:pageblock>   
  </apex:form>
  </apex:page>

public class UserData {
List<Userwrapper> userList {get;set;}
PUBLIC String usr{get;set;}
List<User> results=new List<User>();
public User user;
public ApexPages.StandardController stdCtrl;
public UserData(ApexPages.StandardController controller)
   {
     stdCtrl=controller;

   }  
       
        
  public Pagereference doSearch()
     {
          results = [select Name,Region__c,Email,LanguageLocaleKey,Manager__c from User Where Region__c =:u.Region__c];
            system.debug('USERSLISTFOR REGION$$$$'+results);
      return null;
     }
  public List<Userwrapper> XXX() 
   {
     if(userList == null) {
      userList = new List<Userwrapper>();

     for(User u:results)
      {
        userList.add(new Userwrapper(u));
      }
      system.debug('****ADDEDUSERS****'+userlist);
       }
       return userList;

  }
     //THIS METHOD IS NOT GETTING INVOKED.
   Public PageReference getSelected()
   {
          
        system.debug('ENTERED INTO SELECTED');
        List<User> selectedUsers = new List<User>();
        for(userwrapper usrwrapper : XXX())
        {
          if(usrwrapper.selected== true)
          {
           selectedusers.add(usrwrapper.wuser);
          }
        }
        system.debug('@@@SELECTEDUSERs@@@:'+selectedusers); 
        return null;
  }
   
    Public Pagereference UpdateManager()
   {
        return null;
   }
    
   public List<User> getResults()
    {
        return results;
    }
 
 }

If only the command button is clicked, then the action is invoked however no result is passed.

But if the first action is executed(picklist value selection) and then cliked on command button there the action is not getting invoked.

Please advice.