If you review the Job Report that the dm_LDAPSynchronization job generates when it is run you should be able to search for those specific users that were added. From there you can see how they were handled by the job and also how they were handled in context to the groups on the Documentum Server.
That information should give you an idea of what is going on. If you do not see the users in the report then the issue may be with the LDAP configuration object on the Documentum Server not reaching out to the proper domain on the AD Server or something along those lines.
You may want to re-run the job and ensure that the trace level is 10 to get more detailed information in the report, but a standard level should at least tell you if the users are even being picked up by the job.
"... I have changed the schedule to run daily at a particular time with frequency daily run basis"
I understand that you state that the job ran normally, but my point was that you can check the job report, which is a log of all of the activity that the job performs, and see if the users you are missing are listed in the job report. If they are not listed there then the issue is either with the configuration object not being configured correctly to reach out to the AD Server to pick them up or the AD Server setup. That job report would be the very first thing I would check, because that tells you exactly how to proceed.
If you see no mention of the users in the job report, then that points to the LDAP Config Object not being setup correctly or an issue on the AD Server.
If you see those users in the job report, then the mention should include why the users are not synced.
So please check the job report from DA, by going to the job, right click and select "View Job Report".
Also confirm what DCTM_Guru is mentioning and confirm that the job has ran since you made the changes in the AD.
Are you seeing the LDAP configuration in DA? You should be able to see this object if you have sysadmin/superuser privileges.