local avatar wont display

im using simple local avatar plugin;
https://www.ads-software.com/support/plugin/simple-local-avatars

instruction says
Use avatars in your theme using WordPress' built in get_avatar() function:

so i placed this in my index.php
<?php echo get_avatar( $id, $size = '100' ); ?> on my theme
and then successfuly uploaded my own avatar.

but when i refresh the index page. it still displays the default gravatar