Any chance you are passing in undef for $FileId, $DepartmentId or $ScheduleDttm or the wrong data-types expected by Oracle (e.g. alpha char for an integer file or dept id)?
Binding on a null could be an issue with the SQL (need AND DEPT_ID IS NULL for example vs. AND DEPT_ID = NULL).
Connectivity (and TNS_ADMIN) does not seem to be the issue here if your first invocation of CheckTransmissionHist is successful. Good luck.