When a charge is created, credit card companies and payment processors take many factors into consideration to determine if the charge is valid or not. Failed payments typically happen when credit card information is entered incorrectly or the card is declined. Declines can happen for a variety of reasons other than the availability of funds.
For transactions in the US, Canada, UK and EU, double check that your zip/postcode has been entered correctly and matches your billing address. Incorrect information can cause your transaction to be blocked.
In some cases, your card may be charged and then immediately refunded if your transaction is blocked. This will show as 2 line items on your credit card statement (a debit followed by a credit).