This is because the AD Attribute you have added may be incorrect. 


Example


You have added the field 'Skype' and the data should be pulled from IP Phone in AD:.


Preferences menu > Fields: 




Here is a list of supported standard AD attributes


assistant, c, cn, co, comment, company, department, description, directReports, displayName,
distinguishedName, division, employeeID, facsimileTelephoneNumber, givenName, homeDirectory,
homeDrive, homePhone, homePostalAddress, info, initials, ipPhone, l, mail, manager, middleName, mobile, pager, personalTitle, physicalDeliveryOfficeName, postalCode, postOfficeBox, sAMAccountName, sn, st, street, streetAddress, telephoneNumber, title, url, userPrincipalName, wWWHomePage


Extension attributes 


(extensionAttribute1, extensionAttribute2 etc.) are also supported as well as any schema change you might have implemented. 

If you want add your own custom fields, you just need to tell Xink the LDAP name of the field and the field gets populated.