Welcome to PR, even if it isn't for the best 'first' reason.

As you have some/many images that display properly it first sounds as if some are damaged or corrupted. The thumbnail is frequently a separate file/image embedded in the larger one. This could explain part of the problem. The other is that the CODEC(software program) that decodes the image to generate the thumbnail are often written to read only a portion of the file (like resizing, but via reading every other or 4th or some number of 'lines') and thus if there is corrupted data it skips over it.

You say they are all the same size? Do you mean the files that will not display? Are they a different size than files that display properly? If so that sounds like a known symptom of a corrupted file. If these are all in a non-proprietary format, such as JPEG, TIFF or the like, try a file recovery program to see if it can fix the damage. There are several problems from checksum to eof marker that may be correctable.

One possible problem is that you have a highly fragmented disk. It is a rare occurrence to affect an image but it can happen. Copy one or two files that are not displaying correctly to another device (thumb drive, flash memory(such as a card in a reader), etc. and then see if the file opens correctly from that device. If not go back to the file recovery software concept.

If you have had this problem often, first replace your memory card in your camera or at least format it! I format cards every time I use them (after unloading the photos ).

Let us know how you come out with the problem. Again welcome to Photography Review and hope you will return with better news and share some photos too!