I add new disqus comment plugin in my WordPress blog. I did everything correctly. But my blog posts only showing old, native WordPress comment form instead of disqus comment form.
so please provide suggestion to solve this problem.

I think this problem may happened because of two things.

Type 1: –
Go to Comments->Disqus->Plugin settings
Check “Disqus comments are currently enabled. (Disable)”

Use Disqus Comments on : All blog posts or Blog posts with closed comments only.

Type 2:-
Got to WordPress admin panel -> Appearance -> Editor
Find and open single.php file
check the file contain

<?php comment_form(); ?>


<?php comments_template(); ?>

instead of

<?php comment_form(); ?>

Now check your blog posts.