@joyryde Sometimes Youtube doesn’t provide a proper thumbnail for higher resolution thumbnails. You can try playing around with the thumbnail quality option on the plugin’s admin page. Select the default resolution or the highest resolution, save your changes and then click “Update Posts” to update the metadata of your posts.
You also overwrite the default setting on individual posts: E.g. you can set “Higher quality” by default and if a thumbnail doesn’t show on a specific post, overwrite this setting with “Standard quality”.
When I looked at one of your posts, the video used this fallback thumbnail:
https://i2.ytimg.com/vi/9IcbCUKfy9s/sddefault.jpg
Instead you want to show the standard quality thumbnail because Youtube apparently doesn’t provide a higher resolution version: https://i2.ytimg.com/vi/9IcbCUKfy9s/0.jpg