Do you know why some posts do not show notes?

I noticed this on several blogs, not just mine. Most posts show the associated notes, but some show nothing at all. (I even counted by myself the notes of some posts when I made ​​the Top 15 of my most popular photos.)

Exemple :

This post shows notes.

This post does not.

And this is the same thing in case of reblog. Notes do not appears when reblogging a posts with invisible notes.