My Android YouTube App won't play certain codecs but plays others just fine
You did not mention the device you are using. Your device does not seem to have license/support to play the VP9 codec in its HW codecs.


One workaround is to tricking the YouTube app that you are using a different device which has license for VP9 codecs (Most recent devices does). Unfortunately you need to Root your device to achieve this because it required to modify System Build.prop file or use Xposed modules.
Other option is to use third party or modded YouTube apps like Vanced Youtube which has option to Force VP9 codecs. Use google to download this.