Why are my WordPress scheduled posts still late after I replaced WP-Cron with a 15-minute server cron?

A 15-minute server cron only gives WordPress a chance to process its queue every 15 minutes, so a post scheduled just after a tick can be nearly 15 minutes late even when it’s working normally. If some are much later, make sure the task actually triggers the right WordPress install—usually by requesting its `wp-cron.php`—and only disable page-load WP-Cron after that external task is confirmed. Then inspect the queue with `wp cron event list` and check Site Health. A fatal plugin task, the wrong path or site in a multisite setup, an HTTP request blocked by auth/WAF, or a busy server can keep due events from draining. I’d schedule a test post a few minutes ahead and save the cron command’s exit status/output. If timing really matters, run it every minute; WordPress scheduling isn’t a real-time guarantee.