If you're paying using your contactless prepaid Mastercard® in store you may be asked to do Chip & PIN more often. This is due to new Payment Service Regulations known as Strong Customer Authentication.
So, if your contactless payment is unsuccessful just insert your card and use your PIN. See how to check your PIN.
If you're using Pockit app it will alert you when you need to enter your Pockit card's PIN next time. Please make sure you have push notifications enabled in the app.