The iCloud Drive folder which you can access from the Finder downloads files on demand by default, i.e. not all files may be directly available for syncing. Files that are not downloaded locally still look like the original file in the Finder, but under the hood they are just placeholder files, starting with a period and ending with the .icloud extension.
If you try to sync a file that has not been downloaded locally, SyncTime triggers the download during the file copy phase. To ensure that all files are always downloaded locally and be able to sync them without delays, you can disable the option “Optimise Mac Storage” in the System Preferences, under Apple ID > iCloud.
