It appears that this failure this is due to the size of the schema being reported upon. I have a schema of 1600 tables and when I attempt to generate this in its entirety then the report tool fails without diagnostic, leaving an empty output file.
If a smaller SubView is created for part of the schema then a succesfull result is acheived.
Check the data modeler log file (\sqldeveloper\sqldeveloper\extensions\oracle.datamodeler\log\datamodeler.log) for possible exception.
Is this happening for all output formats? In reports generation dialog there are combo boxes 'Available designs' & 'Available models'. Is the selected model non-empty? In the report generation directory there is file report_data.xml. Is this file empty or just the report file is empty?