Sometimes a reboot of the nowtv box and your Router can resolve a blurry picture, though not guaranteed a reboot will work.
You could try your back button on the nowtv remote and then jump back into the TV programme to see if there is any improvement.
Was you watching Sky Atlantic on the live channel feed or on catchup / boxset ?
Sky Atlantic is looking fine for me on my box.
Are you using the box over wifi or Ethernet ?
If over wifi when you go into the box settings under Network > About, what's the wifi reception signal showing from the box ?
Watching some Sopranos episodes back to back on a Stick, we suddenly hit an episode that was all blurry.
Just exiting the episode and starting it again was enough to bring it back to its usual breathtaking 720p self.