What model / year is your Shield?
I have a 2015 and a 2017 Shield and Optus Sports works on both.
Have you tried to clear data / clear cache / reinstall?
Does the Kodi add-on work for you (not saying you should use it, but just as a test)?
You might have more luck troubleshooting / getting advice over on the Shield and Optus Sports threads on Whirlpool: