thumbnails images not appearing

I have used the following code to show my featured image as a thumbnail, however the featured image/thumbnails are only appearing on my homepage side widget ‘other posts’. If I click on a post they do not appear under ‘you may also like’ or on my side widget ‘other posts’. What can I do? This is the code I found and I’ve entered it into my child theme style.css:
.entry-featured {display:none;}
.widget_ci-latest-posts .entry-featured, .entry-related .entry-featured {display: block;}