Thumbnails Loading And Not Loading At Random

I'm on Windows 8.1. Sometimes, when I open a folder full of image and video files, the thumbnails don't all load correctly. Most of the time, they do. But sometimes, about half of them don't load, or just one or two don't load, and refreshing doesn't necessarily do the trick. It might cause more of the thumbnails to load, or less. It seems to be completely random. This just started happening in the last couple of days. Any ideas why?