This is the advice it gave me.

1/10 rating for a thumbnail with a 6.8% CTR & 43.9% average % viewed over 2 years.

Add expressive faces to the thumbnail. (It had 2 impossibly expressive faces)

The image needs to have good contrast and saturation in the colors (huge red & green highlights against a more dull BG)

Add short, bold and contrasted text on the thumbnail ( The text was "PROS" and CONS")

It's usually better to limit the number of elements in the thumbnail to a handful to make them more visible and eye-catchy (There was 2 people, a background image, 2 words, and a title.)