This is a very long thread, and it would be helpful if you showed us a screen capture of your situation, as was done it that thread.
Yes, it is a very long thread, but symptoms that I have are the same. I would rather not have to jump through all the hoops that the original poster was put through. I have folders and images that aren't being displayed in the Lightroom library. Here is what has been done. I am consolidating all of my pictures onto a NAS device. Some of my pictures were already in Lightroom, but many of them weren't. I moved the images that were already in Lightroom onto the NAS device. When I opened Lightroom it didn't find the pictures (as expected), so I right clicked in the library and selected the folder on the NAS and it found all of the pictures that were in Lightroom. Then I moved all of the pictures that weren't in LIghtroom to the same folder structure. I did an import/add to add the new pictures to Lightroom. Lightroom said that the new folders and images were added, but when I look at the folder structure in the Lightroom library there are missing folders and images. I then did a Sync thinking that the images weren't really added. The sync ran and at the end it said that there are no new images to add. Basically I have a set of folders and images on my NAS device that Lightroom has recognized as being added to the database catalog, but those folders and image files are not showing up in the Library view. I am running Lightroom 5.7.1 on a mac mini that is running Mac OS X Version: 10.12 .
Here is a screenshot from the library in LIghtroom:
Notice how this list starts at 2010-fall. Here is a screenshot from the finder window on the NAS:
Notice how many folders there are before 2010-fall that aren't showing up in Lightroom. These are all folders that were not previously in the Lightroom catalog. I have tried to search for files that exist in these folders using a text search in the grid view. They are not found. I have done multiple syncs and imports, but after the first import/add, Lightroom comes back with a message that says everything has already been imported. These seem to be the exact same symptoms that were reported in the previous thread.
I would rather not have to jump through all the hoops that the original poster was put through.
All we can do is make recommendations based upon the information you have provided us. There are no guarantees that the first thing we suggest is going to solve the problem. Is that okay?
I did an import/add to add the new pictures to Lightroom. Lightroom said that the new folders and images were added, but when I look at the folder structure in the Lightroom library there are missing folders and images.
This very strongly suggests that you have accidentally imported the photos using the COPY option and not the ADD option. You need to search your entire Lightroom catalog to see if the photos are in some other folder. Here are the steps, please follow all of these four steps in order:
1. In the Lightroom Library Module, on the left, under Catalog, click on All Photographs
2. Turn off all filters (Ctrl-L once or twice)
3. Turn off all stacking (Photo->Stacking->Expand All Stacks)
4. Search for at least one of these photos by file name using the Lightroom Filter Bar
Sorry for my earlier ingratitude. I appreciate your help. It turns out that the pictures that I thought weren't added to the catalog, were added to the catalog. I didn't have duplicate photos, but I did have a large quantity of photos that couldn't be found. I tried letting Lightroom find these photos in the new location, but it wouldn't let me do that because the folder structure was changed, so it didn't recognize them as the original files. It was too difficult to try to extract these photos out of the folders that they had been copied into and I wasn't afraid to lose the edits on the files, so I removed all of the missing pictures from the catalog and then did a Sync. The pictures were added and can now be found.