This article is all about fixing your glue, by which we mean, checking the NS records in your domain's DNS zone, and making sure they match exactly the name servers being used at your registrar.
Note: Hatchling, Baby, and Business plans cannot use private name servers, so there is no need to worry about the glue on those plans.
How to change the NS records when using private nameservers.
Be sure this is done for every domain name you host.
Login to Plesk, and go to the desired domain's control panel (if not already there). Under "Websites & Domains", click the "DNS Settings" link. Next to the domain, click the "[Manage]" link.
Here you can edit the existing NS records, or add a new NS record if needed.
Login to Plesk as the admin user. At Home, click the "DNS Settings" link. Here you can edit the existing NS records, or add a new NS record if needed.
After your name servers have been created and the glue is set, if you are using private name servers, then you must add the necessary A records to your domain. (See "DNS Update").