Please know that as an extra layer of security all the cards need to be verified by MasterCard 3D secure program, or Verified by visa in case of Visa cards. If your card issuer does not participate in the mentioned schemes, the card top up may fail.
Apart from that the most common reasons are that you've entered incorrect card details (Expiry, CVV, Name on the card), insufficient funds, or you may have exceeded the daily or monthly top up limit.