Hi,
There could be several reasons.
Please check if you have a valid license, without it the cron will not work. In the top right corner of Acymailing you can see if you have a valid license.
Check if the cron has triggered Acymailing. You can see the last time Acymailing was triggered in Configuration->Queue process. (During troubleshooting you might want to enable Acymailing to send you a report each time Acymailing is triggered)
In case the cron was correctly triggered check if there are items in your queue. If there are items you have a problem with you send process. If there are no items Acymailing probably has nothing to send. Check the settings in your automation.
As for your question about the setting to only send new articles this has not been added to v6 yet. It will come back for sure.