Trying to sync one of the folder content with 4,560 photos already showing in lightroom. When I select "Synchronise Folder..." it shows me 1 new photo and 102 missing photos.
When I select "Show Missing Photos" it list the photos which are really not missing - clicking on "Show in Explorer" gets me to the picture.
When I select "Synchronize" it list 103 photos to be imported. I select "Import" and all looks good however if go ahead and try to sync the same folder exactly the same scenario repeats over and over again.
Please check the images in Library module, if the images are missing then refer:How to find missing photos in Photoshop Lightroom and locate the images.
Once all the images looks good and you are able to edit them in develop module, then you add them in one collection and sync.