Data don't sync. What am I doing wrong?

The second time I lost all my data(200 tabs, bookmark panel, pws, addons). My steps: 1. Sync all data into 1 firefox account from 1 device 2. Reinstall my OS on the same device. 3. Connecting back by logging in and pressing the sync button and... Blah! I see nothing. No synced tabs, no bookmark panel. At the same time I did this kinda operation with having connected 1 account to 2 virtual devices(1 - real OS, 2 - Virtualbox). So could you please explain how Firefox Sync works? Is there a way to use sync account as a keeper for tabs and bookmark panel without using 2 devices? Or other ways? Thanks!