This is setup issue only.
You can run the following script which will help to diagnose the setup issue for the Requisition Approval Process.
R12: PO Approval Analyzer Diagnostic Script (Doc ID 1525670.1)
The possibilities include when the Approver forwards to the other manager who is not part of the Work Flow.
Please check this, Did you make position and approval setups in correct hierarchy that is set up in Document Types setup?
Try checking approval groups amount limit and charge accounts if you are using and Approval assignments whether the document types are assigned to the user position.
This seems to be set up issue