We encountered a strange issue, We are using assign flexi task in our SP2013 Workflow, where task is created perfectly but emails are not triggering to the respective assignees intermittently. Please help us with the solution.

We are encountering this issue with only this workflow so we are not suspecting this would be a SMTP server issue. 
'From' field is not configured in any assign flexi task and 'To' variables are properly configured.
We have already checked the junk mailbox , no respective task emails are received.

Your solution would be much appreciated.