What happens if you access Sky Sports via the Sky Sports News App, then press the OK button on the remote when watching SSN, then navigate to the Sports channel you want to watch ?
Apart from that have you tried all the trouble shooting stuff in this linked guide.
If you have the same issue, then it would seem reasonable that the same help that @schnapps offered in the posting above yours ought to help, no?
If you are having issues with either your NOW TV Box or Stick can you please let us know the version of the app you are running?
You should be able to find this out by heading to My Account.