Photos missing from Lightroom won't import again.

This probably means that the images are already in your catalog, but as a copy in another folder. Initially, when you invoke 'Synchronize Folder', Lightroom will only look at what it knows for this folder (zero images) and compare this to what it actually sees in that folder on disk (243 images). So that is why it says there are 243 images to import. But when you then try to really import them, Lightroom checks the new images against what it has anywhere in the catalog, not just in this folder. And that is when it finds that the images are already in the catalog, just not in this folder but in another folder. 

How were the images initially imported? Did you perhaps place them in this folder and then imported them? If so, then what probably happened is this. When you imported the images, you should have selected 'ADD' at the top center of the import dialog. That only adds the images to the catalog, but leaves them where they are. What you (errogeneously) did was select 'COPY' however. The result was that the images were copied to another folder, and those copies were imported. Those are also the images you worked on.

Go to 'All Photograps', and sort by Capture Date. That should make it fairly easy to find one of these images. Select one, then choose menu 'Photo - Go to Folder in Library'. Lightroom will show you in which folder this image is stored.

In LrC, in the Library Module, looking at the Folders Panel, do you have any folders with a ? mark

https://www.computer-darkroom.com/lr2_find_folder/find-folder.htm