The e-mail alerts get turned off automatically if/when the BookCrossing mail server receives bounces when it tries to send you mail. This is most often caused by your provider regarding BookCrossing's emails as spam. Generally in such cases they are filtered at the server level, so they do not even turn up in your spam box: you just never get to see them, as they are bounced immediately. BookCrossing then switches these alerts to 'off' because it makes no sense to keep sending mail to a non-responsive address.
Is there any way you can notify your email provider that these messages are wanted, and not spam? Can you, for example, whitelist the address email@example.com ?