custom excerpt is not being shown

One thing that can be happening is if you have added a Read more tag in your content it is overriding the manual excerpt. just check and see if you have <!--more--> tag in your content body. if so remove it.

The next thing is the_excerpt()may use auto generated excerpt. it is said in Wp Codex. what you want to use is the get_the_excerpt() method which will retrieve a set custom excerpt.

<?php if ( has_excerpt() ) : // Only show custom excerpts not autoexcerpts ?>
    <div class="exceprt-container"><?php echo get_the_excerpt(); ?></div>
<?php endif; ?>

See get_the_excerpt() reference here