Thumnail pictures on icons in eplorer

Usually Vista will display thumbnails on its folder icons, of images in that
directory or subdirectories. On the whole this seems to work fine. However,
there are a few folders where Vista will not insert them thumbnail images
onto the folder icon. I have noticed that this tends to occur with the
larger folders (i.e. those of around 26GB or larger).

Has anyone else noticed this, and are there any solutions ? Help much
appreciated !