Please help, any help would be greatly appreciated !
"The reason for deletion in the first place is, when I deployed an application on this machine it never came, so, thought of deleting and adding again." - you could've simply removed the machine from the collection rather deletion which deletes from sccm database itself.
Enter the value you want and search all the resources you want to select.
Once the collection is created only a single resource is a member: A query requires that AD Discovery has updated the group memberships in the database (full or incremental – both will suffice) and once that is completed the collection has to be updated.
The reason for deletion in the first place is, when I deployed an application on this machine it never came, so, thought of deleting and adding again.
Are you using query based or direct membership rule?
If direct membership, you can remove the system from there in future.
Can you exit the console and reopen it and try to add the machine again to the collection?
or serach for that machine in " All Systems" collection after reopening? Problem was with the "Collection Limiting" page while adding the workstation, I was browsing for the machine and selecting "all", but keeping it blank did the trick, and I found the machine and could add it ...