Welcome to the Roku Community!
We appreciate you letting us know about the issue with the Youtube channel.
Please make sure to restart your Roku device after removing the channel and before adding the channel back to ensure the process is successful.
If the issue persists with the channel not showing previews, you'll need to contact Youtube Support to report the issue and request additional assistance. They’ll be able to best assist with any issues and functionality inquiries. Many channels on Roku are developed and maintained by the channel provider themselves.
You can reach them here: Youtube help support