Make sure the files are formatted correctly. The proper structure for the CSV/JSON data files is at https://www.postman.com/docs/postman/collection_runs/working_with_data_files.
Another issue that can create problems is the file-encoding. Try opening your data file in a text editor (https://www.sublimetext.com/, for example) and saving the file in different formats (File > Save with Encoding).
If that doesn't help, you can contact us at firstname.lastname@example.org.