Usually a new casual world is lined up to start with a new normal world and they are "tied" together so on the new casual world can be transferred players only from the newly started world.
I am guessing since Casual 11 opened nearly on the same time as W64, transfers to the new Casual world are possible only from us64 atm.
If you can't transfer from that world probably try sending a ticket
Thank you Red for explaining that. But, yes that is true. And you usually can not transfer to a casual world unless you have played in the world for 14 days.