Sometimes your payments might be declined , this could be because:
You could have pending payments or direct debits which impact your available balance.
Some chip and PIN readers may experience connection issues which means the merchant cannot contact Tesco Bank to authorise your transaction.
Depending on the type of card you were issued with at the time of applying for your Tesco Current Account, you may not be able to use your card for offline transactions such as Pay at Pump or Transport for London (you will know if you have this type of card as the seventh digit of the long card number across the middle of your card will be a 5).
This is a safety precaution as some merchants do not check that you have funds available in your account before taking a payment. This could lead to fees being applied to your account and placing you in an unarranged overdraft.