Why is my post's featured image applying to my my blog page too?

This is my first wordpress website here and so far it’ll looking good. The only thing I have left that I just can’t figure out is the featured image for my blog page.

I have set the main blog banner to a default featured image that’s hard coded into the header.php file (and my blog page does not have a featured image so it usually shows the default like i should), which looks like this.

<?php

                // Check to see if we're on a non-home-page and if the non-home-page has a featured image.
                if( !is_front_page() && has_post_thumbnail() ) {

                    the_post_thumbnail( '960, 403' );

                // if we're on the home page
                } elseif ( function_exists('dynamic_sidebar') && is_dynamic_sidebar('banner-widget') && is_front_page() ) {

                    dynamic_sidebar('banner-widget');

                // no featured image set
                } else {
                    echo '<img src="https://www.averylawoffice.ca/Avery-Banner-Default.jpg" alt="Avery Law Office | Summerland, BC">';
                }
                ?>

Which was working fine. Now I’m trying to set individual featured banners for each post but the minute I set one for the newest post the banner on the main blog page changes to that one. However all other later posts do what I want them to do.

Is there something in the header I should change or have to set in single.php that I might be missing? If you need to see a specific code please let me know. I wasn’t sure what page/code to show so I started with the header conditional tag.

Any help or advice is appreciated as always!