Hey. I have tried this, but nothing happens. Any tips?

I have test it in ur site ... the covers are working in ur own Lucid theme .... 

in my theme the covers working not .. they are blocked via CSS ... thats no error

If u like see the covers in my theme than go under my theme CSS and find the code in

line 544 - 546 and delete it

.bx-cover {

  display: none;

}

than save and clear ur caches.