There are a few reasons why your Offer might not have been honored:
Most common issues:
You need to show the redemption page in the app to your server/cashier - they need to see the valid redemption screen
Your Offer might be ineligible if you've used it recently at that restaurant (return window hasn't passed yet) -> The app will not let you use it again so this is rare.
Offers can't be used during Offline Events or combined with other discounts
The Offer is not valid on the day you are trying to use it. You should always review the Availability section at the top of the Offer screen. -> The app shouldn't even let you redeem it so this is also rare.
Less common reasons:
The restaurant may have temporarily closed or had equipment issues
The staff isn't familiar with the Offer. This happens sometimes if staff has turned over at a restaurant or the offer is new at the restaurant. The best course of action here is to be patient and ask to speak to the manager who typically knows about how to apply the Offer to your check.