I don't think so. As emails are sent correctly for the same approver it has something to do with the configuration for the other FF id's. Did you check the two OSS notes? Are those implemented? Did you check if notification is set to "Workflow" for all FF id's?
Also check the MSMP instance. Do you see something there? Use transaction GRFNMW_DBGMONITOR_WD.
Regards,
Alessandro