You're doing this with a Suitelet, so that suggests to me that you have to instantiate a template renderer in Suitescript, and load the Item Fulfillment into the template renderer.
Did you build your template from scratch or base it on an existing one?
The problem likely exists either in how the Item Fulfilment record is being loaded into the template, or the template source code, although I am aware that some fields available in the UI are not available in the record object loaded into the template, I'm not aware of whether those two line item fields have any such exceptions.