Hi, Thom.
Our tickets get sent via wp_mail(). Therefore, you might need to install an SMTP plugin if your tickets are being delivered inconsistently.
Besides our emails not sending, you probably also notice other emails not sending, such as password reset emails.
Here are a few to consider (not a comprehensive list and none are guaranteed or supported by us):
You could also use an email sending provider like https://wordpress.org/plugins/mailgun/ or https://wordpress.org/plugins/mailin/ or other alternatives.
None of these providers are recommended in particular or supported by us, but I hope this information helps you make some progress.
To be clear, email deliverability is almost never an issue with our plugin; it’s a WordPress-level issue or even higher up, such as server settings or your domain or IP address being associated with SPAM (whether accurately or not).
Please let us know once you find out a working solution in case it might help other users, too.
Thanks!