I have just had another go:
I have made sure featured image is set.
I have deleted posts and customisation of frontpage
I have recreated posts (obvioulsy adding their featured image again )and shortcode on frontpage .
No thumbnails – in fact I do not get them anywhere – is there somewhere else I need to do to turn on thumbnails?
If I alter the size of images to make them smaller so they look better when in blog posts they just stretch to fit some preordained size and then pixilate.
Hope you can help – without thumbnails everything is too big