server – Files do not sync smoothly between Dropbox clients

I have installed Dropbox headless clients on 2 servers and I need to keep files in sync, within reasonable delay like a minute or two. I noticed that they are often not in sync.

I made today a script, which writes a timestamp every 5 minutes into a file (control.timestamp) and moves it into Dropbox shared folder. Then script sleeps 60 seconds and tries to read the same file from another server. For now those timestamps are 70 minutes apart. Same time through the web UI I can access the most fresh version of the file, so it propagates into Dropbox in seconds and very well.

Dropbox status on other server says “Up to data”, also on file:

$ filestatus Dropbox/control.timestamp 
Dropbox/control.timestamp: up to date

Restarting Dropbox client seems some times to help, but not always, at least on first attempt.

What could be wrong, that files do not propagate to other client correctly?

NB This is not a question about moving files from one Ubuntu server to other, for this task are much better tools. There are involved Win computers with desktop users who try to send files to the server simplest way I imagined.