Hi Sriram,
I have seen an issue in a case where the response from the AD was limited to X numbers and hence the list was not getting populated. You can confirm this by running below command on ASG against your IWA realm
Login to SSH-Console and from enable mode, execute below command
show xml-realm-search user Realm Searchpath
Realm is the name that you have used.
Searchpath is the domain directory
In the example below, my realm name is DIRECT and Searchpath is BCBLRLAB
The command would be show xml-realm-search user DIRECT BCBLRLAB and give and output as below
Try with this command and see whether the list you are getting is shorter that the expected or not.