Set Item Line Field every time a record is created

Hi i have been trying to set a script that every time an invoice record is created and have a checkbox field not verified; a custom line item field gets the value of the amount field.

I had this code but, when the invoice is created from a Sales Orders it only update line 1.

Does any one knows if i have some errors in this, kind of new in scripting.

function set_custom_amount(type) {        if (type == 'create' || type == 'edit') {            var record = nlapiGetNewRecord();            record = nlapiLoadRecord(record.getRecordType(), record.getId());          var wmedios = record.getFieldValue('checkbox_field');          if ( wmedios == 'F'){              for (var i = 1; record != null && i <= record.getLineItemCount('item'); i++) {                  var amount = record.getLineItemValue('item', 'amount', i);                     record.setLineItemValue('item', 'custom_amount', i, amount);                         nlapiSubmitRecord(record, true);          }          }     }  }