Thumbnail Editing Does Not Work

I am trying to edit the thumbnails that shows up on my homepage. Starting from the edit post page, I click on featured image –> edit image (on right) –> select “thumbnail” on the thumbnail settings box –> crop the image –> save –> “set featured image” –> then hit update back on the edit post page. Still shows original thumbnail on home page, which is off center. What am I doing wrong?