The YouTube video thumbnails are created when you first uploaded your video to YouTube. The availability of a video thumbnail is always up to YouTube. If the video thumbnail is not available, YouTube Showcase WordPress plugin lets you pick another thumbnail of a different resolution.
If you have a video which shows its thumbnail in the Starter edition but not in Pro edition, it means your configuration for the thumbnail resolution for that video is NOT the same in the PRO edition.
The starter edition uses "Medium" quality thumbnail resolution by default. The value is set by Video Image Resolution field.If you set the same resolution in the PRO edition, you will get the same thumbnail.Remember, both plugins get the thumbnails from YouTube.
You were able to display video thumbnail in the starter edition because"Medium" quality or the thumbnail of a certain resolution was there to begin with. What is going to happen if it is not there? The PRO edition gives you 9 options to pick from vs. the starter edition only provides 3.
What's going to happen if the thumbnail exist but not in the same resolution you want? If the resolution is low, the image is going to be blurry or not appealing to your visitors. the PRO edition lets you upload your own image in this case. The starter edition does not have this feature.
In the PRO edition, you can set small and large video thumbnails individually as well. This feature is very handy and lets you display smaller images in the teasers such as navigation thumbnails or widgets. It directly affects the experience of mobile visitors or the visitors connecting to your site using low-speed internet connection. The larger images load slower and take more bandwidth than smaller images.