There are number of reasons for why your credit/debit card may have been declined:
a. You may have gone over your daily transaction limit.
b. You may have entered incorrect card details: invalid card number, expiry date or CVV code. Please verify that these are correct. Please also make sure that your card is still valid and has not expired.
c. No authorization from card issuer - check with your issuer to make sure that your card has been authorised for online transactions and that there are not any protections in place that are preventing you from a successful online transaction.
d. Insufficient funds - amount of the transaction you tried to perform is exceeding funds available on your card.
Please contact email@example.com, if you have any additional questions in regards of declined card deposit.