What can I do if Magento invoice emails are not sent anymore?

If you use our Automatic Invoice Extension for Magento and invoice emails are not sent anymore please check the following points:

Please check the configuration at System › Configuration › Automatic Invoice:
Is the Automatic Invoice Extension activated and are the checkboxes for "Generate Invoice" and "Notify Customer" triggered for the specific payment method?
Has the Magento invoice document been generated for the specific order and is it flagged as "invoice email sent"?
Which Magento Extension is used for the payment method? Perhaps it uses itself a mechanism for generating invoice document and sending the invoice email. Then perhaps it is better to exclude it from the Automatic Invoice generation.
Are there any error messages in the Magento and/or Automatic Invoice log files in var/log/?