This trigger inserts and updates contact fields fine for the first contact but when there are multiple contacts in Task, the task does not update contact fields for the rest of the contacts. What am I doing wrong? Thank you in advance!
 

if (Trigger.isafter && Trigger.isInsert){ 
              ActivityHandlerClass.insertContact(Trigger.New);
     System.debug('I RAN INSERT');

     } 



public class ActivityHandlerClass {
        public static void insertContact(List<Task> tks){ // Trigger.New comes as List here
        Set<Id> ContactIds = new Set<Id>();
        List<Contact> ContactList = new List<Contact>();

        List<Task> taskContacts = [SELECT Id, Subject, (SELECT RelationId, Relation.Name from TaskRelations) FROM Task WHERE id in:tks];
        System.debug('Print Task Contacts: ' + taskContacts);

        for(Task ti: taskContacts)
        {
            for(TaskRelation tRel: ti.TaskRelations)
            {
               System.debug('Print Task Contacts: ' + tRel);
                System.debug('Print Task Contact Size: ' + ti.TaskRelations.Size());
                If(ti.TaskRelations.Size() >0){
                       ContactIds.add(tRel.RelationId);
                  System.debug('Print added to ContactIds: ' + ContactIds);

                }
            }

        }
                List<Contact> relatedContacts = [select id,Last_Contact_Date__c,LastActivityDate, Last_Activity_Subject__c from Contact where id in:ContactIds];
                System.debug('Print RelatedContacts :' + relatedContacts);

        for(Task t :tks)
        {
                for(Contact con : relatedContacts)
                {  
                  If(t.ActivityDate >=con.Last_Contact_Date__c)
                    {
                    con.Last_Activity_Subject__c = t.subject;
                    con.Last_Activity_Date__c = t.ActivityDate;

                    ContactList.add(con);
                         System.debug('Printining that ADDDDDDDDDDDDDDDDDDDDD   ran');  
                         System.debug('ContactList is being printed:' + ContactList); 
                         System.debug('ContactList ID is being printed:***********' + con.id); 
                    } 
                 } 
            System.debug('Update ContactList***************');
        } 

           If(ContactList.size()>0)
           {   
            upsert ContactList; 
           }
  } // End of Insert Method