Some WordPress themes doesn’t have a feature to put an excerpt instead of full posts on home page.
To do this just go to your Appearance -> Editor , then select ‘Content.php’ ( usually at bottom right), then find the below code.
Replace this…
<div class="entry-content">
<?php
/* translators: %s: Name of current post */
the_content( sprintf(
__( 'Continue reading %s', 'twentyfifteen' ),
the_title( '<span class="screen-reader-text">', '</span>', false )
) );
wp_link_pages( array(
'before' => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentyfifteen' ) . '</span>',
'after' => '</div>',
'link_before' => '<span>',
'link_after' => '</span>',
'pagelink' => '<span class="screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>%',
'separator' => '<span class="screen-reader-text">, </span>',
) );
?>
</div><!-- .entry-content -->
to this:
<div class="entry-content">
<?php
if ( is_single() ) :
/* translators: %s: Name of current post */
the_content( sprintf(
__( 'Continue reading %s', 'twentyfifteen' ),
the_title( '<span class="screen-reader-text">', '</span>', false )
) );
wp_link_pages( array(
'before' => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentyfifteen' ) . '</span>',
'after' => '</div>',
'link_before' => '<span>',
'link_after' => '</span>',
'pagelink' => '<span class="screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>%',
'separator' => '<span class="screen-reader-text">, </span>',
) );
else :
/* translators: %s: Name of current post */
the_excerpt( sprintf(
__( 'Continue reading %s', 'twentyfifteen' ),
the_title( '<span class="screen-reader-text">', '</span>', false )
) );
wp_link_pages( array(
'before' => '<div class="page-links"><span class="page-links-title">' . __( 'Pages:', 'twentyfifteen' ) . '</span>',
'after' => '</div>',
'link_before' => '<span>',
'link_after' => '</span>',
'pagelink' => '<span class="screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>%',
'separator' => '<span class="screen-reader-text">, </span>',
) );
endif;
?>
</div><!-- .entry-content -->
Reference
https://wordpress.org/support/topic/wont-show-summary-only-full-post-on-home-page-posts