It’s possible something is happening to your phone (battery low, bluetooth not working, stampwallet not downloaded, internet is down), our app (server issues) or something else. Of course you still want to receive your stamp(s), so this can be a real issue for you.
Don’t worrie. Just ask the cashier to sign the back of the receipt with the number of stamps you should have received. The next time you visit take the receipt with you and claim your earned points.