I have one folder out of dozens where only 1/3 of the photos will import or synch into Lightroom. All the files are .jpgs. When I import, it only pulls 225 of 600+ photos. When I ask it to synch, it recognizes that there are nearly 400 missing photos, but when I tell it to import those missing photos it comes back and says there is nothing new to import. To troubleshoot I deleted the folder out of Lightroom and tried to re-import. I renamed the folder on my server and tried to import. I moved the folder off my server onto my desktop and tried to import. I tried to synch from all those locations, and every time I did it said there were 385 missing files, but when I went to import it thought for awhile and decided there was nothing new to import. I rebooted Lightroom. And now I have no more ideas.
The photos that are not importing have already been imported into your Lightroom catalog. There is no point, and no benefit, to importing them again.
You need to go to the Library module (not the Import screen) and have Lightroom find the photos by filename (or other).