This happens mostly when your ordered item is restricted from receiving online discount. For example: Meal Deals, Buy One Get One., etc as they might have been already offered under discounted price. However, decision of including/excluding the discount for an item lies with the takeaway owner.
Also, few takeaways will not include delivery charges while calculating the online discount and just provide discount for the total value of the items selected.
In case of any further questions, please contact Live Chat Support on the website between 11:30 - 23:00 hours.