Hi Sandeep,
You might want to:
a) Test rule 168 via transaction PFAC_DIS and check which manager it is finding for the relevant user id - if any, and whether they are linked to a user id - as of course they cannot receive anything unless they are linked to a user id
b) Add the_RULE_RESULT to your binding to capture who was found back to a workflow container element - then you will be able to see that in the workflow container via the workflow log without having to go through debugging.
Also I agree with Ibrahim - you need to work with the leave request application, as generally it determines the manager up front and simply passes that to the workflow. If you choose a different agent to the one the leave request application is expecting that could prevent the approver from approving it.
Hope that helps
Jocelyn