Portfolio page doesn’t display featured image

hello,

i’m using “Visual Portfolio, Posts & Image Gallery” plugin with a theme called “Airtifact”. I created a personal layout but when I use the short code on the portfolio page appear the structure but not the thumbnails images. Every portfolio items has a featured image but still in the portfolio page I can only see the structure with grey thumbnails. I already tried to disable plugin that could give problems and I also tried to disable all the plugin not necessary for the theme but it doesn’t work anyway.

thanks in advance for your attention and your help.
best!

buona giornata
buone cose
a presto
graziano