One thought, thought I'm not an Adobe tech. It might be about duplicate images on your disk.
It seems the the "Synchronize Folder..." function that you get when you right click on a folder in LR is unaware of the concept of duplicates. It scans your folder (and subfolders) finds all the images that exist there which don't have an entry in the LR catalog and counts them. Then when you do the import, it discovers that they are all duplicates and you end up with an empty import dialog. [in my experience, the little count number in the upper right of the import dialog counts all the way up to equal your total images in the Sync folder, then counts down to zero].
Now, in that blank looking import dialog, uncheck the "Don't Import Suspected Duplicates" checkbox on the right side of the screen. Poof! You should see a bunch on images appear.
NOTE TO ADOBE ENGINEERS: This would all be obviated if the Import dialog that shows up when you are Sync'ing behaved like the regular import dialog and showed the dup images as greyed out, and not simply showing nothing.
My 2 cents, YMMV.