Look in Folders and not Collections (or vice versa) The setting can be enabled or disabled depending on your preferences but it’s often the cause of photos in subfolders not showing when you think they should be there.
To view the current setting choose the Library menu > Show Photos in Subfolder. This setting lets you control whether or not you see photos in subfolders when you click that folder in the Library module. If you click on a folder that has subfolders but you see nothing or none of the images in the subfolder that is typically caused by a Lightroom setting.
