I saw the RSS feed you have used.
The post does not have any image in the RSS feed. That is the reason no thumbnail is displayed.
Please check the first item – https://ahorroseguro.ec/feed/
<title>¿Cuáles son las diferencias entre el Mercado de Valores del Ecuador y Wall Street?</title>
<content:encoded> has no image at all.
In other posts, the content has images.
If you are interested I can suggest a modification to code where you can have a default thumbnail image when there is no image. Please let me know.