It depends on the theme actually, sometimes the creator of the theme didn’t include the notes html code. If the creator did, then he just blocked it. You can change this in the Customize page > Theme > Enable custom html(under the theme section thingy) and you’ll see the html codes. You have to search the html code where it says “notes”, you have to look good, sometimes it’s really hidden under all those codes. Like in my Tumblr it’s like this:
</div><!/POST>
{block:IfShowNotesOnPermalinkPages}
{block:PostNotes}
<div class=“post” id=“n”>
<h2>Notes</h2>
{PostNotes}
</div><!/NOTES>
{/block:PostNotes}
{/block:IfShowNotesOnPermalinkPages}
You can see mine says “post notes” so I can see my notes.
But sometimes when I go check the notes on my Tumblr blog thus the page that everyone else sees, I don’t see my notes but when I switch to another browser like firefox I can see it. So sometimes it can be the theme creator if he put the code in the html or not, Tumblr’s faulty or you have to check on another browser.
P.S the best way to check your notes is to go to your dashboard and on the right column click on the “x-amount posts” and you’d see all the things you’ve posted and all the notes that everyone else has posted on your post.