Images not resizing

Hiya,

I’m building a site using Genesis framework and Genesis Sample v2.0.1. In my media library I have two images that are 400px by 400px and after adding them to my page I changed the size to 200×200. In the edit view they are the right size however when I preview my page they’re the original size.
If I use the preset thumbnail (150×150) size it works in page but the custom sizes do not.
Any advice or help would be appreciate – does this mean I have to add the custom image sizes for each page to my php file?
Thanks!
Fiona