Hi Eduardo, I checked your yearly post sitemaps a few times. They are really slow and I even saw the 5OO internal server error once.
Make sure you keep your option ‘Split by’ on ‘Month’. Your monthly sitemaps load fast enough and all are correct.
However, even though the response is correct, the status code that is returned by the server is 4O4 which means “not found”. I suspect Google sees this as a problem (and it is!) so it will not treat the sitemaps further.
Do you have any idea why your server or WordPress itself would return a 404 status code for posts or feeds?