emsnotjustcake on "Preview Image not appearing in tweets"

I have just set up my blog and when I tweet the URL on Twitter, I get the preview text but not the image. See example here:

https://twitter.com/EmsNotJustCake/status/727829320937410560

I've tried using the JM Twitter Cards plugin and it looks ok on the post but when I tweet it the image still doesn't appear. Can anyone offer any advice to a total novice?

Thanks

Ok so my latest post works for some reason!

https://twitter.com/EmsNotJustCake/status/727880605506121728

Also I put the link of the post mentioned earlier into Card Validator and it says its all good but the image still doesn't show! I'm so confused!