If you already have saves in the Steam Cloud, at the start of a gaming session you may see an exclamation mark on the cloud icon next to the game button.
This means that synchronization with Steam has not been completed (this error appears when you start the game on a new virtual machine or PC). Click the cloud icon to open the synchronization error window.
Select Retry Sync to force the synchronization of your saves. Wait for the synchronization to finish, and then start the game.
After you finish playing, wait for the synchronization with the Steam Cloud to complete (you can track the progress on the left side of the launcher, next to the game's name).
You can also check whether your progress was synchronized with the Steam Cloud by opening Steam Remote Storage before closing the session.
Why can synchronization fail?
Cloud synchronization depends on Steam's servers. If Steam is experiencing technical issues or carrying out planned maintenance, your saves may fail to synchronize, and other Steam services may be temporarily unavailable.
Steam performs routine maintenance every week on Tuesday at about 4:00 PM Pacific Time — around 23:00 UTC in summer and 00:00 UTC (Tuesday to Wednesday night) in winter. It usually lasts from a few minutes to an hour. We recommend not starting or finishing a gaming session during this time.
If synchronization fails, wait until Steam services are back to normal and click Retry Sync again.