Problem solved! And I think I have stumbled onto something. In answer to your suggestion PinkElephant, I had been trying to do just that - add some text to one of the notes on my phone that wasn't syncing to the web - and watching what happened. I could force that note to sync by doing that, but my problem was - how do I identify the missing 589 notes from the 11,890 total so that I can force them to sync?
And then I noticed something. I keep a weekly journal for my office work, and I was missing my journals between Sept. 9th and Oct. 31. So when I went back to one of those un-synced notes on my phone and added some text to force it to sync with the web, I noticed that ALL of the missing notes in that notebook synced! That lead me to compare the note counts in each of my 81 notebooks on my phone to those on the web, and on my computer. When I clicked on each notebook to verify the note count, it seemed to force a sync for each NOTEBOOK, so that by the time I was finished clicking on all 81 notebooks, my total note count had grown from the 11,890 to 12,483 - matching my phone perfectly!
So I opened my desktop app to see if I could replicate the same thing. Sure enough, before I started, my total note count was 11,890. After clicking on each NOTEBOOK, one at a time, when I was done - my note count was also 12,483.
Not sure why this happened, but I'm glad to have everything synced again. I hope this helps those of you who are also struggling with this issue. Trying clicking on each NOTEBOOK, one at a time, allowing it to sync up. Then check your total note count at the end. Hopefully, this works for everyone.
Thank you to all of you (PinkElephant, Gazumped, Dave in Decatur) who took the time to try to help me out on this issue. I really appreciate the time you took to help a stranger. Much appreciated.