Bhavika,
1. might be issue with Cron not running from time to time.
2. might be issue with mail server- if there are too many emails to sent and server has limitation on number of emails, for example, per hour.
3. Might also be preferences set to "never" or "auto" on user's side.
Troubleshooting:
1. When you set your cron in control panel, you can choose to receive an email each time cron runs. If this option is disabled, enable it. This will help you to monitor whether this is cron related issue.
2. How did you find out that emails were not coming? Which email notifications did you expect to receive? Does this happen with all users?