Hide Feature Image not working

I have also clicked on Hide Feature Image from a Single post and added the following css code

.single.single-post .post-main-image {
    display: none;

but the feature images are still showing up at the top of my posts.

My site is www.shebasjewels.com

Hello, you are not using GreatMag theme so I guess that this issue is not actual. Is it correct?

Kind Regards, Roman.