Contactless payments are often declined because of spending limits or contactless payment limits.
Because no signature or PIN verification is typically required, contactless purchases are typically limited to a set maximum amount per transaction. This is known as a "floor limit". The floor limits may vary by country or by the merchant. In the UK, it's typically £45 though some merchants may only allow less.
Contactless cards need to be authorised occasionally by using CHIP and PIN or by using them in an ATM.
You can do a maximum of 5 consecutive contactless transactions, when you reach this limit you will need to use CHIP and PIN to reset the counter. This limit is set forth by The Second Payment Services Directive (PSD2). You can find more information by visiting https://www.fca.org.uk/news/press-releases/fca-finalises-revised-psd2-requirements
Remember that, once your card's activated, you'll need to use it first with CHIP and PIN to activate contactless payments.
Alternatively, you may have reached our prepaid card limit of max 15 Transactions per day. You can find these limits as part of our fees and limits which can be found here.