There are two posts that have the category 'WordPress' and are not working, one has the following:-
Categories: WordPress
Tags: Plugins, WordPress
The other has these:-
Categories: WordPress
Tags: WordPress
Yet theres another post with the same categories/tags as above, and its working.
It's pretty random. I thought initially it was the posts that had images within the post as all the others that are working don't have images, I tried removing them and this didn't resolve it.