This might sound daft, is the app updated to the latest version?
You could try deleting and reinstalling the app.
Worst case you perform a factory reset to the TV, this often squashes bugs on the system.
Then there is get in touch and report it with Now TV staff
Click “chat online” to speak to Now TV staff
@gavs82008 live channels play 7831 stream quality,tin star is playing 4830 stream quality so it must be a problem with demand programs and yes my samsung smart tv does have latest version as it updates it self and the live sports channels are good as well
As another suggestion (might not work) before going to Live chat to discuss with Now TV staff.
You can always delete and reinstall the app.
Link for Live chat.
Just make sure to click "chat online" within the greenbox