Check the job that is creating the flat files (likely IEBGENER). You will see it is using the SYSVIEW logger exit, GSVXLGEX, to dump the records from the log stream to the GDG.
The parameters on the exit might look like this:
//SYSUT2 DD DISP=SHR,DSN=xxxx.xxxx.xxxx.xxx
The part of this that is extremely important is the NORDW parameter.
If this is not coded then the GDG file does not contain valid records for Report Writer to process. F