Thumbnaills work sometimes - sometimes not?

[howtofeelhappier]

Hi,
I've had a good look through the threads on thumbnails, but can't find the answer to this one, so apologies if it's already been covered.

On my site (work in progress): http://www.howtofeelhappier.com/blog/ I find that some of the home page thumbnails are working, but others aren't, even though I believe I have done the same thing for each of the featured articles.

It's always consistent - i.e. if it doesn't work, it stays not working.
I've tried re-doing some of the thumbnails and have double checked my spelling in the custom field tag.

Any ideas why I'm getting such variable results?

Thank you for any help you can offer!

Clare