Microsoft Windows can be configured to lock and password-protect an idle computer. In Splashtop Remote Streamer version 126.96.36.199 and later versions, you can opt to disable the secure logon (the CTRL+ALT+DELETE sequence) for logging on to Windows. Here is an article from Microsoft that explains how to do it:
So if you are having trouble unlocking a computer via Splashtop Remote, try to disable the secure logon sequence. Please make sure to update Splashtop Remote on both devices to v188.8.131.52 or later.