the_modified_date returns published date

I wanna display modified date instead of published date. I have this code in my theme:

echo '<li><i class="fa fa-clock-o"></i> <span class="post-date updated">' . get_the_date() . '</span></li>';

i changed it to this:

the_modified_date('j F Y', '<li><i class="fa fa-clock-o"></i> <span class="post-date updated">', '</span></li>');

but it still shows the published data.

my post is published on the 1st:
http://img.mywordpresstips.com/date/gyDoK2RG.png

and I updated it on the 7th:
http://img.mywordpresstips.com/date/pnMvCfmu.png

and the_modified_date() function, displays the published date.
am I doing something wrong? how can I fix this?

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