This can have many reasons. Please check if Windows-Explorer shows thumbnails for your images. If it doesn't, WIA-Loader cannot show thumbnails too.

If there are thumbnails in Windows-Explorer, chance are good that WIA-Loader can display them too, but there are still cases where WIA-Loader will be unable to show thumbnails. WIA-Loader uses .NET Framework to create thumbnails, so it depends on the capabilities of .NET to support different image formats. Especially for raw image formats, this support is quite bad.

If you have .NET Framework 3 (or higher) installed, you can try these codecs in order to enhance support of .NET for raw formats.