Running the latest version of LR Classic on a Samsung All-in-One Win10 I5 machine. I used to run LR Classic on this an an older iMac whch was replaced with anewer MacBook Pro. The LR photos and the catalog were on an external drive. Everything worked perfectly up until about June, 2020 when I started having sync problems, i.e., wouldn't sync with my Android phones up or down. When they went to LrC version 10 my MBPro aged out so now its only running on the Win10 machine and the LR Catlog has been moved to the Win10 machine itself. The catalog has about 50,000 plus photos of which some 10,000 are synced. LR Web and all three Android Phones (Moto Z2 Power Android 8; LG V40 and V60 running Android 10) show the same number in LR Mobile All Photos = 10,365 and Deleted = 0) LR Web also shows 10,365in All Photos and 0 in deleted. LR Classic shows ALL Photos of 55,085 and Synced Photos = 10,538 with 267 photos syncing and 12 pending when one opens the sync pane. Those 267 syncing photos have been doing so for at least a month. The rotating blue arrow will stop after a short while. As part of the troubeshooting process I made a couple of new Collections and set them to sync with LR Web and they did not. Likewise editing synced photos either in LrC or LR Web did not propagate the edits up or down.
At the request of Adobe tech support a couple of weeks back I made a new catalog and set it to sync with LR Web. After a day and a half downloading it failed reporting All Photos = 10,292 and Synced photos = 10,208 and claimed it was stil syncing 34 photos. During the download sync process I would occasionally look at the Sync Pane and at one point it claimed to be uploading 11 photos from the empty catalog!
I spent another few hours today chatting with Adobe Technical Support and they on their own made another new empty catalog and started it syncing. I am not happy with them for the moment since they coud not explain how doing the same thing over again would fix anything!
I have rebuilt the Sync data several times, I have renamed the sync data folder a coupe of times, I have search for missing pictures on all devices and on the web. I have on a couple of occasions gone through each pending file downloading it manually from the Web and then deleting it from the web. And of course verified the catalogs too.
The only abberational behavior I have seen has been some files showing up with "TEMP_" prepended to the file names. There is usually a companion file of the same name so I have deleted all those beginning with TEMP_ in LrC and on the Web.
Also my SmugMug LR Plugin seems to have crashed with the advent of the latest LrC version.
I'm mostly replying so you don't feel ignored. Sync issues are a pain and come back for me as well. I've had to rebuild Sync data a few times myself, and there's no real explanation for why it works sometimes and not others. I've saved out metadata, removed and reimported problem files etc. After the new catalog upgrade it's currently okay. As to count differences, I think that videos can sometimes be a differentiator.
I am feeling ignored but only by Adobe Tech Support who failed to call me during the scheduled 9 hour time window today. Of course the new empty catalog they started on the last support call failed, too, just as it did before.
What has set my blood to a boil is that an Adobe Rep closed my Trouble Ticket because it "had been open too long" and immediately opened a new one for the same problem! In one of my lives I was a Field Engineer for IBM providing hands on support at customer locations. If I had ever done something like that I would have been terminated immediately. Conduct such as that certainly slows down the recognition of software problems and lengthens the time to resolve them. Perhaps that is why sync is so iffy.
The solution ended up being pretty drastic, though. After being blown off, lied to, and being forgotten by Adobe Support I was pretty down and just decided that although Adobe software was pretty great I just could not afford the lost time and energy associated with the lack of support so I decided to switch to some other software. As part of that process after pausing sync on Lightroom Classic and turning off sync for each of its collections on my Win10 PC, I deleted all 10,000 plus photos from Lightroom Web along with all the web albums. Then I synced each of my three Android phones and the sync removed the photos from Lightroom for Android on each phone. I then removed any and all Adobe Apps from my phones and from my old MacBook Pro. Then I restarted LrC on the Win10 machine and rebuilt the sync data. At this point I fully expected nothing to happen if I turned sync back on. Well, guess what? When I did turn sync back on it started syncing back the 10,000 plus photos! WTF? Nothing was supposed to be syncing. If anything it should have been counting down to zero from the original 10,000 plus number like it usually did and then back up to the number of yet-to-be-synced photos. So I killed that, made a new empty catalog, opened LrC using that, turned sync on and everything looked good. Next I imported all 55,361 photos from the old catalog without moving them. Turned on sync and nothing synced (Hurray!!!) Then I tried sycing just one collection and that worked, too. The I reinstalled LR on ane Android Phone, took a couple of pics, manually added them to LR on the phone and they synced, too. So it looks like I'm back in business now. My CC Membership doesn't end until April 2021. If LrC stays trouble free until then I may stay with Adobe but I am going to search for alternative workflows between now and then. Ironically, just after I completed all this Adobe Support called me ... sounded like the same guy from before.I was polite but short with him. It will take a lot for them to regain any of my trust.