Incorrect function - when selected to show on the home page, the G+ is not post specific, but general, all the same for all the posts, so like when used for the blog itself, not the posts.
I went through it too. I have it on my site, manually added:
in header.php
<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
{lang: 'cs'}
</script>
in index.php at TWO places after each "read more" code:
<?php echo the_content('More »'); ?>
<g:plusone size="small" a href="<?php the_permalink(); ?>"></g:plusone>
while the first place is for the first post and the second for the rest.