P: Synchronize folder does not import new images unless show import dialog is active

uncheck "dont import suspected duplicates" when you are on the import window. Fixed it for me

HYPHENS ARE THE PROBLEM. 
Try renaming those files that aren ́t being imported, removing the hyphens in the filenames, and it works.

It actually makes it worse as before I could import with the workaround and now I can't import anything.

Adobe should give everyone a couple of free months because I am paying for something that does not work!!!

It seems I had 2 issues:

The first was that for some reason a few photos were imported to a second location, which LR was aware of. This made them legit dupes until I removed the files.

I'm not sure what was preventing the other set of photos from showing up in the sync (or rather, as others have said, they show up when I try to sync, but are later ignored and don't actually import -- LR doesn't say why but presumably they are detected as duplicates). These are files that were originally imported but for some reason LR "lost" them in the catalog (either a partial import or ... I don't know). Since I forced the import by ignoring duplicates, I can no longer replicate the issue. I'll start a new thread if I see this happening again.

> other forum posts seem to suggest that lighroom keeps the info indefinitely, including for deleted photos, theoretically to protect you from re-importing photos you deleted because you didn't want them

That's not true. Lightroom looks for an "Import Hash" in the database, which includes the filename (as it was before the photos were imported) plus the capture date plus the file length. If you remove the photos from the catalog, this import hash is deleted along with the rest of the record. It's also not transferred if you use export as / import from catalog and as far as I know, it's not created for photos that originate via sync.

If you're still seeing issues in 6.3, can I get you to create a new thread please, with a link to this one - simply because this was referring to a 6.2 bug which was fixed, and it's likely you're seeing a slightly different problem. Thanks!

I did a bit more digging and it looks like all of the photos that are not syncing are being flagged as duplicates. However, I can't find any way to tell lightroom to re-generate whatever data it uses to identify dupes (other forum posts seem to suggest that lighroom keeps the info indefinitely, including for deleted photos, theoretically to protect you from re-importing photos you deleted because you didn't want them). Un-checking the "don't import suspected duplicates" import box works around the sync issue but is definitely not a long-term solution.

Definitely still happening with 2015.3 and El Capitan. My photos are located on a network drive. Maybe that's what separates the "fixed" from the "still broken" users?

Hi - I just updated to lightroom cc 2015.3 because the previous version was making it impossible to do anything in lightroom and at the suggestion of the forums I installed the latest version. Now I cannot sync any folder as the above comments say as well. When I right click on a folder it does not show (syncyronize folder( as an option (not listed at all) and when I use the menu options they are greyed out and cannot be selected. HELP!!

Library > Synchronize Folder (greyed out)

I'm using a macbook pro with OS X El Capitan installed.

It happens the same to me
Not happy with this issue..

Strange. I have 2015.3 and still have the issue