Hi Vishal,
How do you maintain the manager for a user? If you are referring to any custom table then check whether table entries are maintained correctly.
You can check the background task by instantiating in SWO1 and executing the respective method.
If you are using the rule for agent determination, you can simulate it using PFAC.
You can find out the cause using the above two transaction for your case.
Thanks,
Karthikeyan