There are a few possibilities.
You are using a third party checkout app for checkouts.
Customers are using Express Checkout (G-Pay, PayPal, Apple Pay, etc)
The Quantity Breaks and Discounts app is not compatible with the items listed above.
There are a few possibilities.
You are using a third party checkout app for checkouts.
Customers are using Express Checkout (G-Pay, PayPal, Apple Pay, etc)
The Quantity Breaks and Discounts app is not compatible with the items listed above.