You should wait 10 hours to make sure that all DC:s are synced.
If you only have one DC or if you are in a lab enviroment you can use -10 so that the sync flag is "ok".
If you use -EffectiveImmediately it will by defaut wait 10 hours to set the sync flag as "ok".
So, to force it to say "ok" right away you need to use "Add-KdsRootKey -EffectiveTime ((get-date).addhours(-10))".
That way you trick your DC to think that you ran the "-EffectiveImmediately" 10 hours ago.