It can be bound to a lot of things:
- Manager who should receive the approval request is not configured to be a workflow user (UserProfile Setting)
- Approving manager does not have a mail address assigned.
- Workflow uses a different assignment logic than you would accept
If I am not wrong ad hoc salary change does not use AWE yet, but the traditional workflow.
You would have to look at the workflow event that are triggered to see which logic is used to obtain the approving manager and check the approving managers workflow settings.