First thing: check your spam folder. Sometimes Formspree emails get in there, specially if you are receiving a lot of them. Mark them as "not spam" if your email provider supports this feature.
Now, if the emails are not in the spam folder, it is because sometimes your email server may return an invalid response to one of our emails, then it is automatically blocked by the email provider Formspree uses. To solve this, please visit https://formspree.io/unblock/<your@email.com> (replacing it with the email address which your form is targeting).