Approvals Mangagement Engine error approver group has no members

I have set up some new rules in AME to handle specific customer invoices.   When invoice is matched to a PO and initiated, the correct approvers show up as FUTURE in the invoice approval history but it never moves to pending.  Workflow show error of approver group has no members.  Cannot understand how this can be when the approvers have been found for the history.  Has anyone else had this problem?