Perhaps these videos are using a HTML5 player. Do you have FFmpeg and perhaps libavcodec packages installed? Patent and copyright restrictions are among reasons why many Linux distros do not come with packages such as FFmpeg in a new install as it provides codecs needed to fully support HTML5 players. As you are using a flavour of Ubuntu so perhaps see https://linuxconfig.org/ubuntu-20-04-ffmpeg-installation or https://help.ubuntu.com/community/RestrictedFormats