I am trying to mass update records using dataloader. Once a record is updated, the trigger should be fired. I tried to update records but the trigger is not fired when used dataloader. When I manually update the record, the trigger is fired. Here's my code:
Trigger
    if(trigger.isafter && (trigger.isinsert || trigger.isupdate))
      {         
            UpdateExpirydate.UpdateCertDetails(Trigger.new);
      }
Apex controller:
   public static void UpdateCertDetails(List<Certification_Attendees__c> lstcertattendees)
{
    
        List<String> cerType = new List<String>{'Certification','Exam Retake','Recertification'};
        List<String> certLevel = new List<String>{'1','2','3'};
        List<Contact> lstcontacts = new List<Contact>();
        Set<Id> contactIds = new Set<ID>();
        if(lstcertattendees != null && lstcertattendees.size() > 0)
        {
            for(Certification_Attendees__c att : lstcertattendees)
            {
            contactIds.add(att.Attendee__c);
            }
            System.debug('ContactIds' + contactIds.size());
            if ( contactIds != null && contactIds.size() >0)
            {
                    for(Certification_Attendees__c attendee : [Select c.Name, c.Level__c , c.Attendee__c,c.Certified_on__c,c.Expiry_Date__c,c.Type__c,c.Exam_Result__c   from Certification_Attendees__c c where c.Attendee__c in: contactIds and
                    c.Exam_Result__c =: 'Passed' and c.Type__c in: cerType and c.Level__c in: certLevel order by c.Certified_on__c desc LIMIT 1])
                {
                     if(attendee != null)
                     {
                     Contact con = new Contact();
                     con.Id = attendee.Attendee__c;
                     con.Certified_On__c = attendee.Certified_on__c;
                     con.Expiry_Date__c = attendee.Expiry_Date__c;
                    lstcontacts.add(con);
                    }
                }   
            }
        System.debug('lstcontacts.size()' + lstcontacts.size());
            if(lstcontacts !=null && lstcontacts.size() > 0)
            {   
                    update lstcontacts;
                 system.debug('Certification details on Contact Records updated successfully');
            }
        }

}

Thanks in advance!