I can play guess as there is no much info provided:
- Vacation rules/delegates, somebody is out and there is a delegate taking care
- Employee-user assignment being not properly set up
- AME (if used) rules to add approval groups besided the ones used hierarchy, e.g. Cost Center/QA/Category approval.
- AME (if used) substitution rules
Just checked the AME rules, it is working fine.. it was an error on the user in understanding the approval chain.