Problem:
The forms' notifications are not sent even if the test email is sent and other WordPress emails are sent.

Solution:
Toolset puts the notifications emails into a queue, then sends it at the end of the page request process. Maybe, we can fix the issue with this form by disabling the queue. Add the following line to your wp-config.php file:

define( 'TOOLSET_FORMS_NOTIFICATIONS_DISABLE_QUEUE', true );

Or add the following line to your child theme's functions.php file:

add_filter( 'cred_notifications_disable_queue', '__return_true' );