2 out of 4 of my expired postcards at the moment went to people who no longer use Postcrossing. They haven’t logged in for months and they were new members at the time.
I’ve seen there’s even a word for this specific type of profile you’re describing: ghostcrossers.
I’m not sure if there’s anything you can do about it. Maybe message them to check if your card has actually arrived? I don’t know. I wouldn’t bother, honestly.
There was a suggestion that postcards travelling to ghostcrossers get automatically registered if they haven’t logged in for more than 6 months, which wouldn’t be a bad idea at all, IMHO: