I am using the default photo viewer on Windows 10. I have been looking through thousands of photos. They are in folders in My Pictures, with about 400-1500 per folder. Especially with the bigger folders, it will show the first few photos fine, but eventually it will skip a bunch for no apparent reason.
For example:
I might have a folder with 800 photos.
Photos 1-200 are shown in order
Only 450 of photos 201-800 are shown.
However, if I start at photo 201, then:
Photos 201-350 are shown in order
Only 350 of photos 351 - 800 are shown
These numbers are made up, they're just an example. I just wanted to make it really clear that it does not depend on the photos themselves. There are no compatibility issues. And it never skips the first few photos that I look at. Wherever I start looking, the issues crop up later on.
I have found that it seems to be more likely to skip photos if my computer is running slowly or if I'm multi-tasking. Not knowing anything about computers, it feels like pictures are queued up in some sort of temporary memory which has limited capacity, and only stores some photos further down the track.
I found this unanswered question on the same topic: