E-mail are not being sent
Assuming that the issue is actually limited to e-mail messages (i.e. the reservations are correctly displayed in
Reservationstab) then the plugin is not directly responsible of e-mail delivery, as it triggers the WordPress core functions to send them, and it uses the best practices to do so. The culprit lies elsewhere, and here are a couple of suggestions to fix it:
Those plugins reconfigure the wp_mail() function to use SMTP servers instead. You can use a very reliable SMTP server like the Google one. This should minimize any server-related e-mail issue.
Read the error.log file of your server (this is a general advice about any issue you may have). If you don't know how to retrieve it, just ask to your hosting provider. Or drop a message to our support.
Check the admin e-mail address that your WordPress general settings displays. It should belong to your domain, otherwise it can be blocked due to security policies by your server.
Last but not least, be sure that the email is not sent for real, rather than thrown into the SPAM folder by the e-mail client.