Hi again Rajesh,
I can unfortunately not see the attached image.
What you describe is also exactly what should happen, as the environmental locks work with mutual exclusion.
If you want the resource PC's to be able to work at the same time, but just not use the same case, I would create an environment-exposed variable which contains an ID tag of the individual case in process - in that way, each resource PC picking up a case can check that non of those environment-exposed variables contain the ID of the queue element which they are picking up.