When I viewed a post on the front-end which has a featured image using that template it generated this markup:
<img src="<em><u>hidden link</u></em>">
I checked in the uploads directory to confirm that the shortcode generates a newly sized image (so it shouldn't be necessary to re-generate them).
Before going any further can you check and confirm the same on your site, to see if there may be some restriction with the hosting that prevents the re-sized image being generated?