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This could be an issue with your WpCron.
You can check with a plugin like Advanced Cron Manager, to see if anything is stuck at the top of the cron queue that may be blocking all the other tasks from executing.
It could also be that your host requires you to setup a manual cron job to hit /wp-cron.php once in awhile.
Or, if it’s a very low traffic site it might be that there just isn’t enough traffic to trigger wpcron. In that case you can use an uptime monitor like uptimerobot, it will visit your site every 5 minutes and that will also trigger the wp cron queue.
Hope that helps. If you have any questions, please let me know!