Why won't Dropbox sync particular files on my mac?

"The HFS+ filesystem includes flags, which provide extra permissions on top of the usual Unix permissions. One of the flags is ""unchangeable"" - which does what the name suggests.

Several files inside my dropbox had been flagged as unchangeable. You can check for this with ls:

Sombrero:directory polleyj$ ls -Ol
total 0
-rwxrwxrwx 1 polleyj admin uchg 0 15 Jan 23:29 file1

-0 tells ls to show the extra flags; uchg indicates that this files in unchangeable.

Sombrero:directory polleyj$ rm file1
override rwxrwxrwx polleyj/admin uchg for file1? y
rm: file1: Operation not permitted
Sombrero:directory polleyj$ mv file1 ..
mv: rename file1 to ../file1: Operation not permitted
Sombrero:directory polleyj$

To fix this, use chflags:

Sombrero:directory polleyj$ chflags nouchg file1
Sombrero:directory polleyj$ rm file1
Sombrero:directory polleyj$ ls
Sombrero:directory polleyj$"

I found another hitch that prevents Dropbox from synchronizing specific files and folders, that isn't mentioned anywhere else: If you use certain special characters like for example backslash \ in the Dropbox folder or file name. See: Certain files won’t sync in Dropbox.

Sometimes you just need to wait. Go to the Dropbox icon in the menu bar and check the status ticker. It will tell you what it's doing. I had a video file with a red cross for about an hour while either Dropbox or El Capitan sorted something out preparing it for upload, then it just started of its own accord. An hour later (big file, s-l-o-w internet), it changed to the blue uploading icon, an hour and a half after that, to a green tick.