If you had enabled Nameserver verification (Verification Mode) and Re-verify Period under the Module's Module Options then you need to make sure the Cronjob is setup properly. Otherwise, any newly added domains will not get verified and existing domains will not be re-verified.
If the cronjob is calling the cron file (/modules/servers/dnsprovider/dnsprovider_cron.php) properly, then you should see a log in the WHMCS → Module Logs (Utilities → Logs → Module Log) every time the cronjob is being run. You can also manually run the cron file in your browser by calling http://yourwhmcsurl/modules/servers/dnsprovider/dnsprovider_cron.php
Conversely, you may disable the verification mode and disable re-verification (Set it to 0).