Workflow is not sending notification emails after using "Until Pause" action

Hi,

We have a very serious problem regarding Nintex workflow 2013.

We suppose to use it to send expiration emails but it seems buggy.

For first few attempts, it

works fine but crazy things started to happen afterwards.

Here is a sample of that workflow.

1. Calculate expiry date

2. Send notification email

3. Wait until expiry date comes ( We have used " Pause Until" action )

4. Send another/Final expiry email

Here are those symptoms: 

Now, no matter how we try but the step 4 is not executing.

If we assign any other task then it is working fine, but only AND only email sending is not working

Workflow has no errors and every action is throwing proper logs and whatever they should do. 

Only emails on step no. 4 are not being sent.

Email setup is fine because emails from step 2 are coming properly.

CAN ANYONE HELP US???