Hi,
When using the frontend you can't use manual sending. When you send a mail it will be added to the queue. Once it's there the automatic sending will handle the sending of your mails. This is done by a cron job that triggers your Acymailing.
Please check if you have enabled the automatic sending in Acymailing->License
The cron will trigger your website every 15 minutes, which is the minimum time interval. So when sending from the frontend it can take up to 15 minutes before Acymailing axctually starts sending the emails.
