How to trigger approval from Child domain request to parent domain user?

[monikagupta]

There is one user belongs to child domain (B) and his manager is belong to parent domain (A).

When user B raising request , it is not triggering approval to his manager A , as manager A belongs to parent domain. 

Please help me on this scenario.

[Tony Chatfield1]

Hi, what exactly do you have configured? What is/isn't happening.
How is the approval managed, flow or workflow? what do you see in the flow\workflow logs.
Can you share details of your investigation and diagnostics?