AutoSSL has stopped renewing certificates

I noticed that a domain on my server had an expired certificate. When I went to check, it looks like AutoSSL had stopped renewing certificates.

What is strange is that there is no error, and no indication of what might be going wrong. When I go to 'Manage users' and run an AutoSSL check manually, it finishes very quickly - much quicker than I remember it finishing. The log simply shows this:

3:43:58 PM The website “DOMAIN”, owned by “USER”, has a faulty SSL certificate (OPENSSL_VERIFY:0:10:CERT_HAS_EXPIRED NOT_ALL_DOMAINS ALMOST_EXPIRED AUTOSSL_READY_FOR_RENEWAL). AutoSSL will attempt to replace this certificate.​

For a new domain that has no certificate yet, it shows:

3:43:58 PM The website “DOMAIN”, owned by “USER”, has no SSL certificate. AutoSSL will attempt to obtain a new certificate and install it.​

There is nothing in, or added to the pending queue.

There is no temporary .txt file created in the root of the domain.

It's as if it says it will check, and then simply doesn't.

Is there any further log that might yield a clue about what's going wrong? Are there any common problems that might prevent requests from being created?

I'm using WHM 64.0, build 32.