Are you finding that when you upload images to your WordPress website that sometimes a thumbnail image is not generated for some images? Check and see if those images happen to be in the .gif format (rather than .jpg or .png).
For some reason, since somewhere around WordPress version 3.5, gif images are being left out of thumbnail generation. The only solution I can offer at this time is to convert those images to JPG or PNG. Good luck.
Let me know in the comments below if this helps you.