Just like regular e-mail, sometimes receiver's e-mail server is down and the card goes to nowhere, some other times it could take a few hours or days to get to the recipient because some servers are slower than others. In other cases you may have mistakenly put in the wrong e-mail address or misspelled it. There is also a slight possiblity that the e-mail could get lost just like what could happen to a snail mail.
To make it up, you can always re-create the card and re-send it to the recipient, we know it can be a bit of a pain.
Some free e-mail providers may deliver our messages to your friends Junk e-mail folder. Please tell your friend to look for your card there, and you should also advise your friend to tell the Junk e-mail filter that they'd prefer to have our messages delivered to their Inbox by using the checkboxes and then clicking the 'Not Junk' button, or something similar.