Hi @aidas85,
Please note that in the free main version, the attachment action is required to create the invoice automatically, so I think something could be happening with your mail sending.
My recommendation is to install the WP Mail Logging plugin and keep an eye on your email sending to see if it is having downtimes. After installing the plugin, go to Dashboard > WP Mail Logging > Settings and activate the Display Attachments switch, to see the attachments’ column in the log list, and track the attachments easily.
Please note that in the Professional extension there is another way to create documents, based on the order status (without the need to attach it), specifically the Create automatically for setting. That is, if you select the Processing or Completed order status for this setting, the PDF document will be created automatically when the order reaches any of these order statuses:
