Issue:- Any New User or Group created in Active Directory is not getting resolved in a specific SharePoint Site collection People Picker. It works fine for other Web Applications and if you create a new Site Collection in the same Web app, People Picker is resolving new Users and Groups there.
For the problematic Site Collection, the old Existing User accounts and Groups are resolving fine, only the New ones are not getting resolved in People Picker.
Get-SPSite "http://SharePoint/sites/collection" | fl > C:\props.txt
If you open the Props.txt file, you will see the properties for this site collection. Look for "UserAccountDirectoryPath" property. By Default it should be blank, if it is not, that is causing new Users in AD not to be resolved in People Picker.
To Change the Property value to Blank, follow as below.
$Site = Get-SPSite "http://SharePoint/sites/collection"
$Site.UserAccountDirectoryPath = $val
Note:- You can also first troubleshoot by creating a new Site collection in Same Web app and check if the issue is similar as per this blog.