Focal point

Contrast, brightness, faces, sharpness and position all compete for attention, and whichever wins is your focal point whether you chose it or not. Composing deliberately means deciding what it should be and then removing whatever fights it.

Thumbnails suffer most when this goes wrong. A simple image with one clear subject is easier to process at feed size than a busy frame full of repeated objects, and the busy version tends to lose. The fix is subtraction: fewer elements, more separation between subject and background, one bright area instead of four. If you cannot say in a word what your thumbnail is of, it has not got one.