Luckily, this doesn't happen very often! If a postcard you've sent is not registered within 60 days of being requested, it expires. The recipient can still register the postcard up to one year from the date it was sent, but in the meantime, you get an extra slot to send another postcard, if you'd like.
We understand that it's frustrating when one of your postcards doesn't reach its destination, but please keep in mind that the postal service is not 100% reliable. Sometimes, mail does get lost, mis-delivered or even accidentally damaged or destroyed. If you send many postcards in Postcrossing, this is likely to happen to you as well at some point — it's part of the postal experience and, unfortunately, Postcrossing has no control over it.
Also, sometimes members get sick, lose access to the internet, have to move suddenly, etc. and postcards end up getting lost. When a member doesn't log in for a long time, Postcrossing automatically sets their account to inactive, and sends them several emails about this issue. I'm afraid we do not have the resources to manually contact every single member who stops registering postcards though... so getting in touch about a postcard that got lost isn't very productive. :(
