More often than not when recovering digital images, typically off a memory card, the image thumbnails are extracted OK, while high-resolution images themselves are not.
There is a phenomenon called "file fragmentation", when the file is placed on the disk in several non-contiguous parts. The sketch showing the process can be seen at the Photo Recovery Limitations page
As a rule memory cards are formatted to FAT or FAT32. If the file gets erased on FAT, only the information how to locate its first fragment remains available. The second and the following fragments cannot be located.
Thumbnail is not big and it is placed near to the start of the file, therefore it can be often recovered successfully, whilst the full-size image is lost. Of all the recovery types, this limitation mostly applies to digital image recovery and unformat on FAT.