Sometimes, when manually copying photos from a computer, or moving them between folders on Android, the system doesn't seem to notice the new files.

The solution to this is to force a "media re-scan". Rebooting the device is supposed to accomplish that, but there's also a nice app at the Play Store which gives one more direct control over it.

Actually, there are several such apps, but the only one I have tried that works reliably for me here is called "media.Re.scan" by Hiroyuki Mizuhara.