Trigger to update field in another object

Question

Here's the trigger I created to update a list of service orders related to a Case when a field is changed in that particular Case. System.debug shows the Order Status value set to 'Closed' but it is not updating in the Service_Order object. Am I missing any important detail?

  trigger InvoiceTriggerfromCase on Case (after update){    
       Case CS = Trigger.New[0];    
       Case oldCS = Trigger.oldMap.get(CS.ID);        
       if(newCS.Invoice_Number__c != oldCS.Invoice_Number__c ){
            List<Service_Order__c> listWO = [Select Order_Status__c from Service_Order__c 
                                            where Case__r.CaseNumber = :CS.CaseNumber];                       
            for(Service_Order__c wo : listWO) {  
                 if(wo.Order_Status__c != 'Closed'){
                 wo.Order_Status__c = 'Closed' ;
                  }                
             }  
             update listWO;   
             System.Debug(listWO[0].Order_Status__c );
             System.Debug(listWO[1].Order_Status__c );
} }

Answer 1

Turns out Order_Status was being reset by another trigger in the third party managed package.
Thank you for pointing out the code not being bulkified. I tried correcting it in the following code. I've yet to create few more triggers and I'd like to learn to do it the correct way.

trigger InvoiceTriggerfromCase on Case (before update) {
for(Case cs : Trigger.New) {     
    Case beforeUpdate = System.Trigger.oldMap.get(cs.Id);        
    if(cs.Invoice_Number__c != beforeUpdate.Invoice_Number__c) {         
        for(List<SVMXC__Service_Order__c> listWO : [Select SVMXC__Order_Status__c from SVMXC__Service_Order__c 
                                                where SVMXC__Case__r.Id =: cs.Id]){       
            for(SVMXC__Service_Order__c wo : listWO ) {  
                wo.SVMXC__Order_Status__c = 'Closed';  
            }  
            update listWO; 
        }
    }   
  }    
}

Answer 2

Try changing your trigger to run as a before update trigger instead of an after update.

Answer 3

I've tried it several ways but still no update occurs..Please help

trigger InvoiceTriggerfromCase on Case (before update) {
for(Case cs : Trigger.New) {     
    Case beforeUpdate = System.Trigger.oldMap.get(cs.Id);        
    if(cs.Invoice_Number__c != beforeUpdate.Invoice_Number__c) {
        System.Debug('previous invoice number is: ' + beforeUpdate.Invoice_Number__c);
        System.Debug('current invoice number is: ' + cs.Invoice_Number__c);
        List<SVMXC__Service_Order__c> listWO = [Select SVMXC__Order_Status__c, SVMXC__Case__c from SVMXC__Service_Order__c 
                                                where SVMXC__Case__r.Id=: cs.id];        

        //System.Debug('Id :' + listWO[0].ID);            
        //System.Debug('Id :' + listWO[1].ID);            
        //System.Debug('Id :' + listWO[2].ID);            
        //System.Debug('Id :' + listWO[3].ID);

        List<SVMXC__Service_Order__c> wo = new List<SVMXC__Service_Order__c>();

        for(SVMXC__Service_Order__c word : listWO) {
            //System.Debug('wo ID' + word.ID); 
            word.SVMXC__Order_Status__c = 'Closed' ; 
            wo.add(word);
        }  
        Database.update(wo); 
    }
} 

}