This happens mostly when your ordered item is restricted from receiving an online discount. For example Meal Deals, Buy One Get One, etc as they might have been priced at a discounted rate.
However, the decision of including/excluding the discount for an item lies with the takeaway.
Also, few takeaways will not include delivery charges while calculating the online discount and just provide a discount for the total value of the items selected.
Please note all Foodhub codes have a maximum cap of £5 on redemption.