I have a trigger for two custom objects (Extended_Warranty__c & Unit__c) that works perfectly within SFDC. The trigger's function is to find the Order_No__c and Product_Code__c field on the Unit__c object and match it to the Ext_Warranty__c and Product_Code__c fields on the Extended_Warranty__c object. This work perfectly when i edit records within salesforce, but when i tried updating records from the Data Loader or new units are pushed to SFDC via API. Not all of the records are assigned to the correct Order_No__c - Extended_Warranty__c. I was wondering if anyone has heard of this issue. Any help would be greatly appreciated. 

trigger TriggerUpdateUnitExtWarranty on Unit__c (before insert, before update) {
  
  // Step 1: Create a set of all extended warranties of Units to query
  Set<String> extwrnty = new Set<String>();
  
  for (Unit__c newUnit : Trigger.new) {
    if (newUnit.Order_No__c != null) {
      extwrnty.add(newUnit.Order_No__c);    }
  }

  // Step 2: Query for all the Units in Step 1
  List<Extended_Warranty__c> potentialproduct = [SELECT Id, Name,Product_Code__c FROM Extended_Warranty__c
                                 WHERE  Name IN :extwrnty];

  // Step 3: Make a Map that lets you search for units by extended warranty
  Map<String, Extended_Warranty__c> ewrntyMap = new Map<String, Extended_Warranty__c>();
  for (Extended_Warranty__c w : potentialproduct) {
    ewrntyMap.put(w.Product_Code__c, w);
  }

  // Step 4: Get the matching extended warranty in the Map by extended warranty!
  for (Unit__c newUnit : Trigger.new) {
    
    if (newUnit.Ext_Warranty__c == null) {
      Extended_Warranty__c ordnm = ewrntyMap.get(newUnit.Product_Code__c);

  //ordnm is the name of the extended warranty record that is associated with the unit.      
    if (ordnm != null) {
      newUnit.Ext_Warranty__c  = ordnm.Id;
      }
     
    }
  }

}