Some of the most common reasons are that you've had insufficient funds, input wrong card details (expiry date, CVC, billing address), or used an expired card. Please contact your card issuer directly as they might have declined our authorisation request for various reasons.
We also have a daily deposit limit for recently added cards, so please wait until the next day if you have just added a new card and added money successfully with it before. This daily limit will be increased automatically after a few deposits. If you need more funds before your limit has been increased, you can add money by bank transfer.