Thanks for letting us know about the issue that you are experiencing.
Is NFL the only channel that you observe this issue with? It's possible there may be an issue within the channel itself that the service provider will need to investigate.
Try removing the NFL channel from the Roku home screen by navigating to the channel tile, pressing the * key on your remote and choosing 'Remove channel'. Then restart your device from Settings>System>System restart. Once your player starts up again, add the NFL channel back once more. Does this clear up the issue you are seeing?