If the notification assistant has stopped sending notifications, it may be due to several reasons. Look into the following action points to get the issue resolved:

Check if You Need an "Executing User"

Searches happen in the context of a user. Depending on how your permissions are configured, you may need to configure an Executing User. If a notification is not sending:

• Try accessing a test issue
• Once you know you can view the test issue, add your own username to the Executing User field
• Test the notification again to see if it sends what you expected
• If it does, then you need to identify a user who will have access to all possible issues that the notification may send out and add them to this field

You have an Event triggered Notification that is not firing

Make sure that you only have a single Event firing in your Transition's Post Functions

Emails started showing columns you didn't expect

Your Template has a set of configured fields:

But the email you get has more fields that expected:

Notification Assistant for Jira 6.4 added support for showing changes in notifications with a Time based trigger. You can disable it by editing the Template and unchecking the Include Field Changes option.

You have provided a hardcoded email, but they're not being notified

You have provided an email like fake@example.com to be notified. However, the user is not getting notifications. Or you have had a notification with this configuration, and it suddenly stopped working. 
Check your users and see if anyone has this email assigned to them. If they do, and their account is disabled, then Jira will not email them.

Check the "Scheduler Details" for Jobs

Check the JIRA email configuration

• Ensure you've properly configured an SMTP Server. Send a Test Mail inside the SMTP Server configuration setup screen. Make a note of any error that is returned from the test.
• Check your JIRA application log files and the application server log files for Out of Memory errors. Typically, the log file will show java.lang.OutOfMemoryError: Java heap space. This has been known to cause the service responsible for sending emails out to fail until JIRA applications are restarted. You should further troubleshoot your memory issues using this guide.
• Check and ensure the Mail Queue Service is installed. Click Admininstration > Services to inspect that the service exists, and is set at a reasonable interval. This interval controls how frequently the mail queue is processed. You can flush the mail queue to send out pending messages immediately to your mail server.
• Inspect your Mail Queue under Administration > Mail Queue. See if you are given the option to Bypass currently sending mail. A stuck email or trackback ping can hold up the queue.
• Check that your Base URL is set to a domain / IP which your SMTP server will accept. Example: Google apps accounts must have a matching base url to their Google Apps domain. 

• Enable additional logging in Administration > System > Troubleshooting and Support > Logging and Profiling by setting the following to DEBUG to see more robust logging about services running at the background:

  • com.atlassian.jira.service

  • com.atlassian.jira.service.services.DebugService

Slack Channel Connection Troubleshooting

• Make sure that you can connect to the Slack web hook from your Jira server

  curl -X POST -H 'Content-type: application/json' --data '{"text":"Testing"}' https://hooks.slack.com/services/<EXAMPLE>

  
  

• If Jira connects to the internet via an outbound proxy like so

  -Dhttp.proxyHost=proxy.example.org -Dhttp.proxyPort=8080 -Dhttps.proxyHost=proxy.example.org -Dhttps.proxyPort=8080 -Dhttp.nonProxyHosts=localhost

  Then you can also test using the proxy from the command line as well

  curl -X POST -H 'Content-type: application/json' -x <proxyUser>:<proxyPassword>@<proxyHost>:<proxyPort> --data '{"text":"Test"}' https://hooks.slack.com/services/<EXAMPLE>

  
  

Capture and Provide Logs

See the instructions at Capture and Provide Logs