Hi, this had me scratching my head for a while. I had a problem with LR5 not picking up jpeg's I'd downloaded from Apple's iCloud. So, for example, any photos downloaded from the iCloud onto my Windows 10 desktop wouldn't appear in my LR Library when I Synchronized my Desktop folder. The solution was to open those files into MS Paint click on the Save icon then re-synchronize. I assume there must be something about the downloaded jpegs from the iCloud which means they're not recognised by LR5 unless they're re-saved.