Hi loksurya,

No configurations are needed, because notifySessionClose and unsubscribe should always be called.

Please consider that there should be no exceptions to this behavior; this is because a successful notifyNewSession is always eventually followed by notifySessionClose; unless notifyNewSession fails.
What can happen is that the session is not closed immediately but a few seconds must pass before a configurable timeout occurs.

As for unsubscribe, please note that this is only called when there are no more active subscriptions for the Item.
This means that if the same Item is subscribed by 10 clients only when all 10 have closed their subscription the relative unsubscribe will be called.

However, if despite the above it seems to you that the issue persists, you should collect the server log of one of these cases after setting these levels (lightstreamer_log_conf.xml):

<logger name="LightstreamerLogger.subscriptions" level="DEBUG"/>
<logger name="LightstreamerLogger.requests" level="WARN"/>

Regards,
Giuseppe