Thumbnails fail when they look cluttered or do not clearly show what the video is about within the first second of someone seeing it. Using small text that becomes unreadable on mobile screens makes people scroll past without understanding your message. Faces with expressive emotions tend to grab attention better than generic stock images or landscapes. Your thumbnail might also blend in with everything else if you use the same colors and style as thousands of other creators. Test different designs by checking which ones get higher click rates and then create more thumbnails following that successful pattern. What do you think makes you personally click on a video thumbnail?