Double scanning can easily be done by mistake. If a ticket is being denied and you're positive it shouldn't be, check the information on the denied screen. It's possible it was already scanned by accident just seconds prior!
When scanning, hold the ticket at least six inches away from the scanning device. Calmly moving the ticket into the view of the device's camera often works best (we actually say the further, the better).
If the device is having a hard time detecting the QR code, try entering in the ticket's code manually. This can be done above the "TAP TO SCAN" box.
If the customer has a digital ticket on their own phone or tablet and isn't being detect, ask the customer to increase the brightness of their own screen.