YouTube shorts no longer supports thumbnail. That being said, if you're uploading the shorts video directly through your YouTube application, you will be able to select a video frame from your content and use it a shorts thumbnail. Otherwise, if you're uploading the video from PC, YouTube will by default select one of your video frames and add it as a thumbnail.
This simply indicates that you're not ranking for these tags… I'd recommend targeting long tail keywords rather than shorts keywords, as they're easier to gain authority and ownership than the one you're using. Short-tail keywords are too competitive, and big YouTubers or content creators already have ownership of those content. Hence, as a small channel owner, you need to focus on those keywords or topics that can narrow down your content further that assist or aid you in gaining authority over the topic. Furthermore, it also depends on how well your video did, audience retention, CTR, etc.
