Problems with Updating text fields

I currently have a php page that uses a MySql connection to display orders in a repeat table. Each row is an order and can individually be updated (each row is an individual form) The editable fields are in textboxes that are initially filled with the value that is currently in the database. For some reason when the "cost" field is not changed (the initiall value is still there) and I update the record the value is lost and that field goes blank.... Any ideas why this might be happening? Any help is appreciated. Thanks.