The feature image is not appearing in the email even though I have selected it in the settings

Hi,
I love the plugin and it is very easy to use. I do have one problem that the feature image of a post isn't being included in the weekly post email even though i have selected it in the settings panel. Instead all I get is a grey box. The feature image works everywhere else on the site so not sure why it isn't coming through.
Any help would be great.
Thanks