I think this is something on the router end. I've added a second device as a test and it started populating data almost immediately.
Sometimes it takes a little while to get enough data to start graphing too...
Check for the last received net-flow in summary page.
Whether have you managed the sources,
Check if you have forwarded the traffic over port 205 which is default
Whether you have enough space in NTA FSDB
Finally install wire shark and check if you are receiving the flow.
Let me know the results.
Try to check is NetFlow configured correctly or correct interface is selected.